US3395619A

US3395619A - Hydraulic propping apparatus with automatic overload protection means

Info

Publication number: US3395619A
Application number: US511645A
Authority: US
Inventors: Grebe Konrad
Original assignee: Individual
Current assignee: Individual
Priority date: 1965-08-23
Filing date: 1965-12-06
Publication date: 1968-08-06
Anticipated expiration: 1985-08-06

Description

K. GREBE HYDRAULIC PROPFING APPARATUS WITH AUTOMATIC OVERLOAD PROTECTION MEANS Filed DSC. 6, 1965 FIG'I HYDRAULIC PROPPING APPARATUS WITH AUTMATIC (WERLOAD PROTECTION MEANS Konrad Grebe, Auf dem Nutzenberg 1, Wuppertal-Elberfeld, Germany Filed Dec. 6, 1965, Ser. No. 511,645 7 Claims. (Cl. 92-51) ABSTRACT F THE DISCLOSURE Hydraulic propping apparatus consisting `of two telescoping prop members having an intermediate tubular member disposed therebetween and a movement-resisting means disposed between the intermediate member and the outer propping member. The intermediate member is hydraulic fluid actuatable both with respect to the inner propping member and the outer propping member and will `move outward in response to an inward overload movement of the inner propping member.

This invention relates in general to hydraulic propping apparatus, and more particularly to a hydraulic propping apparatus adapted for propping mine roofs and featuring self-contained automatic overload protection means.

Essentially, the propping apparatus of the invention `comprises an elongated outer propping member, an elongated intermediate tubular member telescopically disposed within the outer prop member, an elongated inner prop member telescopically disposed Within the intermediate tubular member, and means for resisting relative axial movement 'between the outer prop and intermediate tu- 'bular members.

Under normal operating conditions, the outer prop member is supported upon the ymine tioor land can be conveniently mounted on a pedestal or footing if desired. The inner prop member is extended by the action `of pressurized hydraulic uid in the nature of a hydraulic actuator piston into a position of propping engagement with the mine roof. Ordinarily, a cap member is mounted to the upper end of the inner prop member for distributing the force exerted thereby over a larger roof area than if said upper end were to be directly bearing against `the roof. However, such pedestal and cap member elements are not directly concerned with the present invention.

When the inner prop member is extended and just contacts the mine roof, -relatively little force is transmitted to the roof, and under static equilibrium, the corresponding opposing force of rooi:` resistance transmitted to the inner prop is likewise relatively small. As the inner prop is extended further, the equilibrium forces increase sharply, depending upon the local penetration resistance of the mine roof.

This propping force is maintained yby means of -hydraulic pressure acting against the inner end of the inner prop, and is equal in magnitude to the product of the area of said inner end and the iluid pressure acting thereupon.

In extending the inner prop, pressurized hydraulic iluid is introduced into a chamber dened by the closed inner end of the outer prop member, with a passage therethrough being provided for the purpose. Since the intermediate tubular member is disposed in slidable sealing engagement with the outer prop member and the inner prop member is disposed in slidable sealing engagement with the tubular member, the volume of this chamber will depend upon the axial positions `of the inner prop and intermediate tubular member relative to the outer prop, with the inner ends of both the tubular member and inner prop lbeing exposed to the full chamber pressure.

3,395,6i9 Patented Aug. 6, i968 As hydraulic iiuid is introduced into the chamber, the inner prop will extend until it engages the roof, with the tubular member being restrained by the resisting means against outward movement Iby the fluid pressure.

The resisting means is set so as `to prevent relative movement between the tubular member and outer prop until a given limit hydraulic pressure is reached. This limit pressure corresponds to a selected limit propping force to be exerted by the inner prop, and hence, below this limit force, only the inner prop moves outward with respect to the outer prop.

Upon reaching the limit propping fonce, the flow of hydraulic iluid into the chamber is shut off so that the amount of uid within the chamber remains constant `absent any leakage, and the inner prop will remain indefinitely at such limit propping force due to the inherent incompressibility `of hydraulic fluid unless the load exerted by the mine roof changes.

With the hydraulic fluid confined within the chamber, the inner prop `ordinarily cannot retract even if the portion of the mine roof which it supports were to break loose from overlying strata which `originally contributed to its support. Such occurrences, called rock bursts, cause overload problems in prior art propping apparatus because if the roof should be partially deprived `of its original support by such a breakage or cleavage in the overlying strata, but not to the extent where it falls down, an additional load is transmitted to the inner prop resulting in an increase in chamber pressure. In such cases, if the inner prop could be kretracted slightly, the increased load acting thereupon could Ibe reduced to the desired limit load without any serious danger of cave-in.

However, such prior art propping apparatus cannot perform such a pressure release automatically, and `thus when a rock burst occurred, `often the roof prop would be dangerously overloaded.

The propping apparatus of the invention overcomes this disadvantage by means of its intermediate tubular member yand resisting means elements. In the event of an increased load acting upon the inner prop beyond the predetermined limit load, the tubular member will lbe displaced outward With respect to the outer prop thereby allowing the inner prop to retract under the influence of the load force until it assumed a propping position corresponding to limit load. Upon restoring the inner prop to a limit load condition, further outward displacement of the tubular member with respect to the outer prop will not yoccur because the hydraulic pressure in the chamber under such limit loading is not suticient to push the tubular member `out against the effect of the resisting means.

In the event of a subsequent overload, the tubular member will again 'be displaced outward to permit retraction `of the inner prop into a position -of limit load.

In accordance with the invention, the inside of the intermediate tube which receives hydraulic uid, is opened towards the closed end or foot of the outer prop. Thus there is present only a single pressure chamber in which the hydraulic iluid acts against a relatively large area corresponding to the full eiective cross sectional area of the inner end of the inner prop, and at the same time acts against a substantially smaller area corresponding to the eifective cross sectional area of the inner end of the intermediate tubular member. This hydraulic pressure tends to force the inner prop out of the intermediate tubular member and also to force the intermediate tubular member out of the outer prop. These forces acting on the intermediate tubular member and the inner prop Will be in a ratio corresponding to the ratio of their respective cross sectional areas.

The movement of the inner prop is limited by the counterpressure exerted by the mine roof, and in order to limit the movement of the intermediate tubular member, a collar is attached thereto which cooperates with stop members on the outer prop.

The invention permits, however, axial displaceability between the collar arranged on the intermediate tubular member and two stops arranged spaced apart from each other on the outer prop for the purpose of thereby achieving a buing overload protection against rock bursts. This takes place in the manner that upon the setting of the inner prop, a pushing out of the intermediate telescopic member into its extreme position is prevented by a resilient resisting or locking device so that in case of a sudden increase in the pressure within the chamber as is caused by a rock burst, the intermediate tubular member can still be forced out further by a certain amount from the outer prop, overcoming the effect of the resilient resisting means.

The basic concept of the invention resides in the fact that the intermediate tubular member, the inner portion of which receives hydraulic fluid, is open with respect to the outer prop, and has a collar displaceable axially between two stops of the outer prop.

The resisting means can be of a type, such as for example, a friction member operatively connected to the intermediate tubular member for movement therewith and disposed in operative engagement with the outer prop member to provide a frictional force opposing their relative movement in both the inward and outward directions, or the resisting means can include a spring member disposed in operative engagement with the intermediate tubular member and with the outer prop member to provide a resilient force opposing only the outward movement of the tubular member with respect to the outer prop.

It is in itself known to take up rock bursts by means of a spring body, the spring force of which corresponds to the setting load of the prop. In this connection, one embodiment of the invention provides a built-in spring body which causes the intermediate tubular member to function as a hydraulic pressure relief valve of relatively large flow cross sectional area. For example, as soon as the rock pressure exceeds the setting load, such las 40 tons, the build-in spring body responds. By the compressing of the spring body, a valve defined by grooves on the tubular member and a conically flared portion on the outer prop member is opened to permit overpressured hydraulic fluid to liow out of the chamber. Thus, according to this embodiment of the invention, the relative axial displaceability of the tubular member and outer prop can be utilized in combination with a spring body resisting means to provide relief valve action simply by providing one or more open axial grooves extending lengthwise from the inner end of the tubular member and depthwise into its exterior surface disposed for communication with an open conically are interior portion of the outer prop whenever an outward displacement of the tubular member corresponding to a predetermined excess load occurs.

lSuch a valve arrangement inherently assures valve closure whenever the prop load falls below the valve operation limit since the spring body will automatically return the tubular member to its normal position wherein the axial groove or grooves are not in communication with the conically flared portion of the outer prop, therefore enabling the intended setting pressure of the inner prop to be normally maintained.

Since conventional pressure relief valves which might be used in conjunction with propping apparatus have only a relatively small cross sectional flow area because of the very high pressures which they must withstand, such valves cannot accommodate the rapid release of hydraulic uid required to overcome the effects of rock bur-st overloads, whereas the relief valve action provided by the propping apparatus according to the invention can accommodate the rapid release of hydraulic iluid pressure necessary to prevent rock bursts from overloading the prop.

In the case of the hydraulic prop according to the invention, because of the relatively small cross sectional area of the tubular member as compared to the inner prop, a relatively lighter spring can be used for holding the tubular member in place against a given hydraulic pressure. For example, with an eifective cross sectional area ratio of 8:1 between the inner prop and tubular member the hydraulic pressure force acting upon the inner prop will be 8 times that acting upon the tubular member for any given hydraulic pressure. If, for instance, 4() tons is contemplated as the prop setting pressure, the inner prop with the cap or like associated with it is braced over the outer prop with 40 tons force against the mine roof. Under such conditions, the hydraulic fluid within the chamber urges the intermediate tubular member outward with respect to the outer prop with Vs of this force, i.e. with a force of 5 tons. This means that only a counterforce of 5 tons rather than 40 tons is necessary in order to prevent an axial displacement of the intermediate tubular member with respect to the outer prop.

This counterforce can be provided for example, by means of a blocking device, formed by one or more friction members resting against the outer wall of the intermediate tubular member and against the inner Wall of the outer prop, so as to prevent any axial movement of the tubular member with respect to the outer prop as long as the blocking force is not overcome by a hydraulic force of more than the limit prop setting force multiplied by the area ratio between the tubular member and inner prop, which for the case of a prop setting limit load of 40 tons and an area ratio of 1:8, would be 5 tons` Alternatively, this resisting force can be provided by means of a resilient member, such as for example a spring acting against the collar of the intermediate tubular member so as to urge said tubular member into the outer prop with an equivalent resisting force. With such an arrangement, the intermediate tubular member returns in each case automatically to its normally retracted position as soon as the inner prop, by reason of the release of hydraulic fluid through valve action is relieved of the excess pressure corresponding to the overload.

The first mentioned solution using a braking device such as a friction member which blocks axial movement in both directions excludes the automatic return of the intermediate tubular member into its normally retracted position, but also oiiers the advantage that it is possible to use the intermediate tubular member as a continuously adjustable prop extension within its limits of travel.

Both embodiments of the hydraulic propping apparatus according to the invention react to a rock burst in the following manner; As soon as the propping load has been increased by a rock burst, the force which strives to lift the intermediate tubular member out of the outer prop increases in the same ratio. By reason of this increased force, the eiiect of the resisting device is overcome and the intermediate tubular member is so displaced with respect to the outer prop that its collar moves in the `direction toward the outer stop of the outer prop. With this movement, the chamber volume available for the hydraulic fluid increases correspondingly. For example, if the cross sectional area of the inner prop is 8 cm.2 with 8:1 area ratio, the cross sectional area of the tubular member will be l cm.2 so that for an outward axial tubular member displacement of 4 cm. the inner prop is retracted 0.5 cm. under the inlluence of the roof load. lf, due to this retraction of the inner prop, the load thereupon is returned to the limit setting pressure, the axial movement of the intermediate tubular member with respect to the outer prop will terminate since the resisting means is set to prevent further outward displacement of the tubular member.

Since overloading of the prop can be avoided in this manner, the intermediate tubular member and the outer prop wal-1 thicknesses can be lighter than would be permissible in the case of prior art hydraulic props. This is particularly favorable in that with a thinner tubular member wall thickness, the ratio between the inner prop end area and that of the inner end of the intermediate tubular member is increased, thereby reducing the control forces which must be exerted by the resisting means for a given prop setting limit load.

If a resisting means capable of bi-directional action is used, such as for example slide friction members, the intermediate tubular member can be used simultaneously for extending the effective propping travel range of the inner prop.

Either a single friction member extending around the periphery of the tubular member, or a plurality of such friction members arranged along side of each other around the periphery of the tubular member can be supported and constrained between collars of the intermediate tubular member axially spaced apart from each other. With such an arrangement, the outer prop can be held fast with respect to the intermediate tubular member in any desired axial position between the lower limit position where the lower collar of the tubular member rests against the lower stop of the outer prop and the upper limit position where the upper collar of the tubular member rests against the upper stop of the outer prop. A buffering type overload protection against rock bursts is furthermore provided as long as the inner prop is not sunk all the way into its inner limit position within the intermediate tubular member, and the tubular member is not extended all the way out of the outer prop into its upper limit position.

If desired, the reaching of either of these two limit positions can be sensed by any suitable conventional means, such as for example, a limit switch, and a visual or audible signal can be initiated thereby, as well as the simultaneous transmission of an equivalent signal to a remote monitoring station which in the case of a mine, can be either above ground or at a gross heading passage.

To use the intermediate tubular member as an extension prop, it can be displaced with respect to the outer prop in the following manner: To extend the propping range of the inner prop, it is rst fully retracted into the tubular member, such as by releasing hydraulic pressure. In the fully retracted position, the inner prop and tubular member are locked together for outward movement in unison, such as for example, by means of one or more drift pins extending through the tubular member and into the inner prop. In the locked condition, the inner prop and tubular member are forced out together from the outer prop simply by admitting pressurized hydraulic fluid into the chamber until they are at a desired extension position. By releasing the locking means, the propping apparatus is restored to its normal mode of operation wherein axial movement of the tubular member relative to the outer prop only occurs when an overload has been reached. In order to reduce the propping range of the inner prop, it is fully retracted into the tubular member and said tubular member is pressed into the outer prop against the resistance of the friction members and by allowing the hydraulic fluid in the chamber to discharge therefrom. This pressing can be accomplished by any suitable conventional means.

However, the propping apparatus can be also operated with conventional extension props and the resisting means can be a spring member which bears against the collar of the tubular member and against the outer stop of the outer prop. In its normal position, the spring, in accordance with the example selected, has an initial preloading of 5 tons or preferably somewhat higher, for instance 5.5 tons. If the spring is pressed as a result of hydraulic pressure increase caused by a rock burst, the restoring force which it exerts to move the tubular member back into its normal position with respect to the outer prop will simultaneously increase by an amount corresponding to the spring compression by the outwardly displaced tubular member.

With such an arrangement, the movement of the tubular member with respect to the outer prop, can with particular advantage, be employed for opening and closing a valve having a large cross sectional flow area. For this purpose, in accordance with another embodiment of the invention, the outer prop is provided with a conically ared portion on its interior, which, when the tubular member collar is placed on a stop facing the foot of the outer prop, comrnences at a seal member disposed between the tubular member and outer prop and terminates at the collar of said tubular member which is provided on its periphery between the seal and the pressure chamber defined by the closed end of the outer prop with axially extending grooves. This seal member therefore moves outward during the opening motion of the valve gate which actually is the tubular member, which motion is brought about only by a prop overload of such magnitude that the tubular member is displaced outward past the axial position where the conical are of the outer prop commences. Under such conditions, hydraulic uid is released from the pressure chamber into the conical Hare and beyond same, preferably out of the entire propping apparatus itself.

The grooves which extend axially in the direction toward the pressure chamber of the propping apparatus serve in this connection as discharge channels for hydraulic fluid f-rom the pressure chamber.

It ris therefore, an object of the invent-ion to provide a hydraulic propping lapparatus having overload protection means.

Another object of the invention is to -provide a hydraulic propping apparatus yas aforesaid which has a quick-acting `automatic overload protection means capable of relieving prop overloads resulting from rock bursts of the type experienced in mine Kroof propping applications.

A further object of the invention is to provide a hydraulic propping apparatus as aforesaid in which the range of prop extension can be selectively varied.

A further object of the invention is to provide a hydraulic propping apparatus as aforesaid tin which the overload protection means is operable with control forces relatively small in comparison to the prop load force controlled thereby.

Still another and further object of the invention is to provide a hydraulic propping apparatus as aforesaid wherein the overload protection means can be adjusted for operation with various different prop limit loads.

Other and further objects and advantages of the invention will become apparent from the following detailed -description and yaccompanying drawing in which:

FIG. l is an elevation View, partly in section, of a hydraulic -p-ropping apparatus according to -a preferred embodiment of the invention.

FIG. 2 is an elevation view, partly in section, of a hydraulic propping apparatus according to another ernbodiment of the invention.

Referring now to FIGS. 1 yand 2 which respectively represent two hydraulic propping :apparatus embodiments A and A of the invention, they essentially differ as to the type of resisting means used for inhibiting axial movement of the intermediate tubular mem-ber 2 relative to the outer prop 1.

In both embodiments, the inner prop 3 is substantially similar in construction, and is shown in its fully retracted position in FIG. 1 and in an extended position in FIG. 2.

The intermediate tubular member 2 is provided with a single collar 4 (FIG. 2) or with two

collars

5, 5 spaced apart `from each other (FIG. 1). The outer prop 1 has a stop member 6 against which the collar 5 in FIG. 1 or in the case of FIG. 2, the collar 4 bears against when the tubular member 2 is fully retracted into the outer prop 1. An upper stop member 7, which :is preferably adapted to be screwed into the upper end of the outer prop 1 is provided to limit the outward movement of the tubular member 2 so as to prevent it from being pulled completely out of the outer prop 1. In the case of the propping apparatus A, the stop member 7 bears against the upper collar of the tubular member 2 when said tubular member 2 is at its outer limit position. In the case of the propping apparatus A', the stop member 7 serves as a retain-ing abutment for a spring member 8 and thereby likewise limits the maximum extension of the tubular member 2 beyond the outer prop 1.

In the apparatus A, one or more friction members 9 arranged between the collars 5 `and 5 provide resistance to any relative axial displacement between the tubular member 2 and the outer prop 1.

Relative axial displacement between the inner prop 3 `and tubular member 2 can, `as illustrated in FIG. l, be prevented by introducing `drift pins 10 through bushings in the tubular member 2 and into corresponding recesses provided in the inner prop 3. This, of course, is done in order to utilize the capability provided by the apparatus A for extending the effective propping range of the inner prop 3 by displacing the tu-bular member 2 outward with respect to the outer prop 1. After such extension of the tubular member 2 has been effected, the drift pins 10 are removed for normal 'automatic overload protection operation.

The pressure chamber of the propping apparatus A, A is provided with :a feed line passage 12 for the introduction of hydraulic uid therein, said feed line 12 being adapted to be closed by a non-return valve (not shown) which is adjusted to the setting load of the propping appa ratus A, A.

The pressure sealing between the intermediate tubular member 2 and the outer prop 1 is effected by a packing member 13 and the fluid sealing between the inner prop 3 and the tubular member 2 is similarly effected by a packing 13'. Outside the packing 13, the outer prop 1 in accordance lwith FIG. 2, is provided with a conically flared portion 14 which is connected by bore holes 15 with a -groove closed by an elastic ring 16. Between the packing 13 and the pressure chamber 11, the outer wall of the intermediate tubular member 2 which is guided slidably on the inner wall of the outer prop 1 is provided with axially extending blind grooves 17.

From the foregoing, it can be appreciated that both embodiments A, A of the hydraulic propping apparatus according to the invention feature an elongated outer prop 1, an elongated intermediate tubular member 2 telescopically disposed within the outer prop :in slidable sealing engagement therewith and arranged for limited axial movement relative thereto, an elongated inner prop 3 telescopically disposed within the intermediate tubular member 2 in slidable sealing engagement therewith for limited axial movement relative thereto and relative to the outer prop 1, and resisting means disposed in operative engagement with the outer prop 1 and with the inte-rmediate tubular member 2 to forceably resist relative axial movement therebetween.

The resisting means can be in the form of friction members 9 as in the case of the lapparatus A, or such resisting means can be in the form of a spring 8 as lin the case of the apparatus A.

In both cases, the outer prop 1 has a closed end portion defining a hydraulic pressure chamber 11 disposed in communication with the inner end of the inner prop 3 and also with the inner end of the intermediate tubular member 2, with a passage 12 being provided through the closed end portion of the outer prop 1 and disposed for connection to a source of pressurized hydraulic fluid (not shown) to introduce such fluid into the pressure chamber 11 for effecting outward axial lmovement of the inner prop 3 into yan extended propping position against the effects of a load force such as the resistance encountered when propping a mine roof (not shown).

In both embodiments A, A', the resisting means is disposed to prevent axial movement of the intermediate tubular member 2 with respect to the outer prop 1 Whenever the hydraulic pressure force acting on the inner end of the tubular member 2 is below a predetermined value which corresponds to the selected limit prop setting load. When the hydraulic pressure Within the chamber 11 is above this predetermined value, the resistance of the resisting means will be overcome and the tubular member 2 will be displaced outward. Thus, when the load force acting against the inner prop 3 increases to the extent whereby the hydraulic pressure within the chamber 11 exerts a force against the inner end of the tubular member 2 in excess of such predetermined value, the tubular member 2 will be displaced outward with respect to the outer prop 1 to permit retraction of the inner prop 3 under the inuence of the increased load force into a propping position in which the load force is reduced to a level corresponding to a hydraulic pressure force equal to said predetermined value acting on the tubular member 2.

In general, it is the effective cross sectional area of the inner end of the tubular member 2 which determines the magnitude of force available for counteracting the iniiuence of the resisting means for any given chamber 11 hydraulic pressure. The force available for propping action is determined by the elfective cross sectional area of the inner prop 3. Hence, for a given prop setting load, the hydraulic pressure within the chamber 11 is fixed, and likewise the hydraulic force acting on the tubular member 2 being dependent thereupon, is also fixed. For operation in accordance with the invention, the resisting means, whether it be the friction members 9 or the spring 8 must exert a counterbalancing force on the tubular member 2 in order to prevent it from displacing outwardly when the inner prop 3 is set for the intended propping load.

On the other hand, in accordance with the invention, it is necessary that the tubular member 2 be displaced outward `upon the occurrence of a propping overload. Therefore, in the case of the embodiment A, the friction member 9 must be so constructed as to permit the tubular member 2 to move outward relative to the outer prop 1 upon the occurrence of an overload. Preferably, in design of the friction members 9, a certain overload tolerance should be allowed, so that tubular member 2 displacement does not occur with negligible overloads.

For a given resisting means construction, the inter mediate tubular member 2 is constructed so that its inner end has an effective cross sectional area selected in relation to that of the inner end of the inner prop 3 to effect outward displacement of the tubular member 2 against the influence of the resisting means at a chamber 11 hydraulic pressure corresponding to a selected propping load force. It has been found to be generally satisfactory in the case of propping apparatus A, A used in mine roof propping applications, to construct the tubular member 2 so that its inner end has an effective cross sectional area approximately s that of the inner end of the inner prop 3.

In the case of the embodiment A', the resisting means is a spring member 8 disposed in operative engagement with the collar 4 of the tubular member 2 and also with the retaining collar 7 attached to the outer prop 1 so as to provide a resilient resistance to the axial movement of the tubular member 2 relative to the outer prop 1. In this embodiment, the intermediate tubular member 2, inside surface of the outer prop 1, and the spring 8 function in the manner of a spring loaded pressure relief valve, and for such purpose, the tubular member 2 is provided with at least one and preferably a plurality of open axial grooves 17 extending lengthwise from the inner end of said tubular member 2 and depthwise into the exterior surface thereof in the portion of said tubular member 2 which is normally disposed for sealing engagement with the outer prop 1, i.e. the inner end portion of the tubular member 2. The outer prop 1 is provided with a conically flared interior portion 14 disposed for communication with the axial grooves 17 to accommodate release of hydraulic fluid from the chamber 11 whenever the tubular member 2 is actually displaced by a distance equal to that between the forward blind ends of the grooves 17 and the beginning of t'he ared portion 14 as measured under conditions of the intended limit prop load.

Accordingly, whenever the load force acting upon the inner prop 3 exceeds the limit value, the increased hydraulic pressure in the chamber 11 acting upon the inner end of the tubular member 2 will actually displace outward with respect to the outer prop 1 against the effect of the spring 8 to position the axial grooves 17 in communication with the conically flared portion 14 to release hydraulic uid from the chamber 11 thereby enabling the inner prop 3 to retract under the inuence of load force to a propping position wherein the load force is reduced to the limit value. As the inner prop 3 retracts and the load force is being reduced to the limit value, the tubular member 2 will be retracted by the spring 8 to its normal position wherein the axial grooves 17 are out of communication with the conically ared portion 14 and the chamber 11 will be sealed at a hydraulic pressure corresponding to the limit load force so as to maintain the inner prop 3 in a propping position corresponding thereto.

The radially disposed passages or bore holes 15 extend through the outer prop 1 and are disposed in communication with the conically ared portion 14 and also in communication with the exterior of the outer prop 1 to discharge therefrom such hydraulic fluid as may be released when the axial grooves 17 are in communication with the conically ared portion 14.

By providing a groove 18 extending around the exterior periphery of the outer prop 1, and the elastic sealing ring 16 disposed in sealing engagement with the groove 18, which communicates with each of the radial passages 15, the entry of dirt into the propping apparatus A can be prevented since the sealing ring 16 will elastically expand to permit efux of hydraulic fluid discharged through the radial passages 15 and will normally seal such radial passages in the absence of such fluid discharge.

As can be appreciated by the artisan, the propping apparatus A can provide overload relief protection through retraction of the inner prop 3 enabled by outward displacement of the tubular member 2 alone, without the release of any hydraulic fluid from the chamber 11, or by the outward displacement of the tubular member 2 combined with the release of hydraulic fluid, depending upon the lengths of the grooves 17 which establish the length of tubular member 2 displacement before the upper ends of these grooves 17 communicate with the ared portion 14.

In a particular application, the grooves 17 can be extended upward to provide a predetermined tubular member 2 outward displacement before hydraulic uid discharge occurs, so that a limited -inner prop 3` retraction is enabled without iiuid discharge to permit limited overload relief. In such case, an overload beyond that limit will result in the tubular member 2 being displaced until the grooves 17 communicate with the ared portion 14, thereby enabling further retraction of the inner prop 3 by reason of uid discharge from the chamber 11, without any significant further outward displacement of the tubular member 2.

Of course, if desired, the grooves 17 can be extended in relation to the flared portion 14 so that the valve defined by said grooves 17 and flared portion 14 cracks open upon a slight overload, thereby providing overload protection by fluid discharge alone.

What is claimed is:

1. A hydraulic propping apparatus which comprises an elongated outer prop member, an elongated intermediate tubular member telescopically disposed within CTI said outer prop member in slidable sealing engagement therewith .for limited axial movement relative thereto, an elongated inner prop member telescopically disposed within said intermediate tubular member in slidable sealing engagement therewith for limited axial movement relative thereto and relative to said outer prop member, and resisting means including a friction member operatively connected to said intermediate tubular member for movement therewith and disposed in operative engagement with said outer prop member to provide a frictional force opposing the relative movement of said tubular and outer prop members, said outer prop member having a closed yend portion defining a hydraulic pressure chamber in communication with the inner end of said inner prop member and with the inner end of said intermediate tubular member, said closed end portion having passage means disposed for connection to a source of pressurized hydraulic iluid to introduce such uid into said pressure chamber to thereby elfect outward axial movement of said inner prop member relative to the outer prop membe and into an extended propping position against the effect of a load force, said resisting means being disposed to prevent axial movement of said intermediate tubular member relative to said outer prop member when the hydraulic pressure force acting upon the inner end of said tubular member is below a predetermined value and to permit such movement when such hydraulic pressure force is above said value, whereby when the load force acting against the inner prop member increases to the extent whereby the hydraulic pressure within said chamber exerts a force against the inner end of the intermediate tubular mem-ber in excess of said predetermined value, said tubular member is displaced outward with respect to the outer prop member to permit retraction of said inner prop member to aid in reducing the increased load force acting thereupon.

2. The hydraulic propping apparatus according to claim 1 including releasable locking means disposed for operative engagement Iwith said intermediate tubular member and with said inner prop member for locking them together for movement in unison.

3. The hydraulic propping apparatus according to claim 1 wherein the inner end of the intermediate tubular member has an effective cross sectional area approximately one-eighth that of the inner prop member.

4. The hydraulic propping apparatus according to claim 1 wherein said friction member is retained between a pair of axially spaced collars on the intermediate tubular member for movement therewith, and wherein said outer prop member has a lixed means defining an axially spaced pair of internal collars disposed for operative engagement with corresponding collars on the intermediate tubular member to limit the axial movement of said tubular member relative to said outer prop member.

5. A hydraulic propping apparatus which comprises an elongated outer prop member, an elongated intermediate tubular member telescopically disposed within said outer prop member in slidable sealing engagement therewith along a predetermined axial length portion thereof for limited axial movement relative thereto, an elongated inner prop member telescopically disposed within said intermediate tubular member in slidable sealing engage- 'ment therewith for limited axial movement relative thereto and relative to said outer prop member, and a spring member disposed in operative engagement with said intermediate tubular member and disposed in operative engagement with a retaining member supported by said outer prop member to resiliently resist axial movement of said tubular member relative to said outer prop member, said outer prop member having a closed end portion defining a hydraulic pressure chamber in communication with the inner end of said inner prop member and with the inner end of said intermediate tubular member, said closed end portion having passage means disposed for connection to a source of pressurized hydraulic uid to introduce such hydraulic fluid into said pressure chamber to eiect outward axial movement of said inner prop member relative to the outer prop member into an extended propping position against the effect of a load force, said intermediate tubular member having at least one open axial groove of predetermined length extending lengthwise from the inner end thereof and depthwise into the exterior surface portion of said tubular member normally disposed for sealing engagement with the outer prop member, said outer prop member having a conically ared interior portion disposed for communication with the axial groove in said tubular member to accommodate release of hydraulic uid from said chamber when said tubular member is axially displaced a predetermined distance outward with respect to said outer prop member, whereby when said load force acting upon the inner prop member exceeds a predetermined value, the resultant increased hydraulic pressure in said chamber acting upon the inner end of the tubular member will axially displace same outward with respect to the outer prop member against the eifect of the spring member to permit retraction of the inner prop member to aid in reducing the increased load lforce acting thereupon, said tubular member being capable of outward displacement until said axial groove therein is positioned in communication with the conically ared portion of the outer prop member to release hydraulic uid from said 4chamber thereby enabling further retraction of the inner prop member under the inuence of the load force to further aid in reducing same.

6. Hydraulic propping apparatus according to claim 5 including at least one radially disposed passage extending through said outer prop member in communication with the conically ared interior portion thereof and in communication lwith the exterior of said outer prop member to discharge therefrom such hydraulic uid released when the axial groove of the intermediate tubular member is in communication with said conically flared portion.

7. The hydraulic proppng apparatus according to claim 6 including a groove extending around the exterior periphery of said outer prop member, and an elastic sealing ring member disposed in sealing engagement with said peripheral groove, said groove being in communication with each radial hydraulic iiuid discharge passage extending through said outer prop member, and said sealing ring being disposed for elastic expansion to permit elux of hydraulic fluid discharged through such radial passages and to normally seal same in the absence of such iluid discharge.

References Cited UNITED STATES PATENTS 1,548,182 8/1925 Burgin 92-52 X 1,723,131 8/1929 Burkholder et al. 92-51 X 2,438,285 3/ 1948 Houldsworth 92--51 X 2,746,254 5/1956 Lucien 92--51 X 2,778,627 1/ 1957 Sands 92-52 X 3,010,752 11/1961 Getfner 92-51 X 3,172,340 3/1965 Blenkle 92-117 MARTIN P. SCHWADRON, Primary Examiner.

I. C. COHEN, Assistant Examiner.

Patent No. 3,395,619

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION August 6, 1968 Konrad Grebe It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shovm below:

Column 5, line 38, "gross" should read cross Signed and sealed this 13th day of January 1970.

(SEAL) Attest:Y

WILLIAM E. SCHUYLE Edward M. Fletcher, Jr.

Commissioner of P A A' nts Attesting Officer