GB2049612A - Docklevellers - Google Patents

Abstract

A dockboard hold-down device comprises a hydraulic cylinder assembly 18 connected between the dockboard 4 and the dockleveller frame 5 which assembly must thus expand or contract to permit the dockboard to be moved. The cylinder assembly has a connecting conduit which permits fluid movement between the stem and base cylinder chambers. A valve means 19 controls this fluid movement. There is provided means for mechanically opening the valve means to permit the dockboard to be raised. When such mechanical opening means is inoperative, the valve means is operative to (1) close if there is a greater fluid pressure in the stem chamber than in the base chamber (thus locking the device), and (2) open if there is a greater fluid pressure in the base chamber than in the stem chamber (thus permitting the cylinder assembly to contract). <IMAGE>

Description

SPECIFICATION Docklevellers The present invention relates to docklevellers, and in particular to a hold-down device used to lock the dockboard at a given elevated or horizontal or declined position.

Docklevellers are used to provide a ramp between the deck of a truck and the loading bay of aware- house or the like, in order to facilitate loading or unloading. Docklevellers provide both a compensation for the height differential between the two levels and a bridge for the gap existing therebetween.

Various types of docklevellers have been proposed. Forthe most part they include a base frame mounted in a recessed pit on the loading bay, a dockboard pivoted at its inner end to the base frame, and a dock-raising mechanism. In this manner, the dockboard is movable between a horizontal crosstraffic position, wherein the dockboard is flush with the loading bay, an elevated loading position wherein the dockboard extends to a vehicle platform higher than the loading bay, and a lowered, declined position wherein the dockboard extends to a vehicle platform which is lowerthan the loading bay.

In order to more effectively bridge the gap intermediate the dockboard and the vehicle bed, a lip extension is hinged to the front end of the dockboard. The lip extension is movable between a vertical hanging position and a near horizontal position respectively as the dockboard is raised between its horizontal and elevated positions. For this purpose a lip extending mechanism is employed such as described in U.S. Patent No. 3,685,076. In this case the lip extending mechanism is actuated by the dock-raising mechanism so as to extend the lip only when the dockboard is raised higher than the level of the vehicle bed. The dockboard is then lowered to its loading position, at which point the now horizontal lip contacts the vehicle bed.

The dock-raising mechanisms may be manually or mechanically operated, or may utilize a hydraulic or pneumatic cylinder. They may operate directly between attachment sites on the dockboard and base frame, or between the base frame and a lever-link mechanism. A dockboard powered by a hydraulic cylinder operating between the base frame and a lever-link mechanism is described in U.S. Patent No.

3,694,840.

Today warehouses are often constructed by development companies wishing to avoid the additional expense of the power cylinders in the docklevellers. This change-overto the mechanically powered systems has presented problems in the industry. Whereas the use of the power cylinders provides a built-in hold-down device in the docklevellers, the use of the mechanical raising mechanisms requires a separate reliable hold-down.

In most mechanically operated docklevellers springs are used to provide a continuous lifting force to the dockboard. In orderto lock the dockboard either at its horizontal cross-traffic position, at an elevated loading position, or at a declined loading position, the locking function of the hold-down device is required. The hold-down must additionally be releasable to allow the dockboard to move between positions.

Previously proposed hold-down devices used in the spring-actuated docklevellers are mechanical, employing complex ratchet devices or friction bars to perform the locking and releasing functions.

These mechanical hold-downs however are subject to slippage and other mechanical failures.

According to the invention, there is provided a hold-down device for use in a dockleveller wherein a dockboard is pivotally mounted at its inner end to a base frame for rotation about said pivot between cross-traffic, fully raised and declined positions and wherein mechanical dockboard raising means are provided to rotate the dockboard upwardly from the declined and cross-traffic positions to the fully raised position, said hold-down device comprising a hydraulic cylinder assembly having a stem end and a base end and comprising a piston element, having a piston head, and a cylinder element within which the piston element reciprocates, said assembly being mountable so that one of the elements may be interconnected with the dockboard and the other element may be interconnected with the base frame, whereby the cylinder assembly may expand and contract respectively in response to the dockboard being raised and lowered, said cylinder assembly having a fluid-filled stem chamber and a fluid-filled base chamber, the volume of fluid in the base chamber when the cylinder assembly is fully expanded being greater than the volume of fluid in the stem chamber when the cylinder assembly is fully contracted, said piston head being so arranged to prevent fluid movement between the chambers, conduit means interconnecting the stem and base chambers, valve means controlling fluid movement through the conduit means between the chambers, and first means for mechanically opening the valve means to permit the cylinder assembly to be expanded as the dock-raising means raises the dockboard, said valve means being operative to close if there is a greater fluid pressure in the stem chamber than in the base chamber and the first means is inoperative, thereby preventing expansion of the cylinder assembly, and said valve means being operative to open if there is a greater fluid pressure in the base chamber than in the stem chamber, thereby permitting the cylinder assembly to contract Preferably, the valve means is a check valve such as a ball and seat check valve and the mechanical opening means is a manually operated lever and piston assembly which can be actuated to unseat the ball.

In another preferred feature, the base chamber includes a reservoir section in order to provide a volume of fluid in excess of the fluid volume contained in the stem chamber. This reservoir section is provided by a housing surrounding an inner cylin der, the inner cylinder being open-ended at the base end so that a continuous chamber is provided.

The reservoir section preferably contains a com pressiblefluid body, such as a pocket of air, which combines with the hydraulic oil otherwise filling the stem and base chamberto provide the 'fluid' previously referred to. The compressible body, when compressed, assists in generating a greater fluid pressure in the base chamber than in the stem chamber, which pressure differential is sufficient to open the check valve. Additionally, the pressure generated by the compressible body increases the flow rate from the base chamber to the stem chamber, enabling the cylinder assembly to be rapidly contracted and subsequently locked at a desired position.

Normally closed valve means are preferably provided to control a second conduit connecting the stem and base chambers. Such normally closed valve means, which may be a spring-loaded ball and seat check valve, are adapted to open when the pressure in the stem chamber reaches a predetermined level. This safety release system is provided to permit fluid movement from the stem chamber to the base chamber when the fluid pressure in the stem chamber is greater than the normal operating pressures.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a schematic view showing a hold-down device in accordance with the invention, in a locked contracted, cross-traffic position; Figure 2 is a schematic view showing the holddown device being expanded with the mechanical opening means actuated to hold the valve means open, thereby allowing the dockboard to pivot upwardly; Figure 3 is a schematic view showing the holddown device fully expanded with the spring contracted and the mechanical opening means inoperative; Figure 4 is a schematic view showing the holddown device being contracted by a man standing on the dockboard, the valve means being open; Figure 5 is a schematic view showing the holddown device in a locked condition at a loading position, the valve means being closed;; Figures 6, 7, 8 and 9 are side sectional views showing the hold-down device incorporated in a dockleveller in the cross-traffic, fully elevated and declined and elevated positions, respectively; Figure 10 is a perspective view ofthe hold-down device incorporated in a dockleveller; Figure 11 is a perspective view of the hold-down device having cut-away portions of the hydraulic cylinder assembly and the valve means; and Figure 12 is a side sectional view of the hold-down device.

The hold-down device ("hold-down") to be described is provided for use in a dockleveller. The hold-down is connected with the dockboard and dockleveller base frame and is functional to lock the dockboard at a given horizontal, declined or elevated position, in opposition to a continuous upward force imparted to the dockboard by mechanical dock raising means (such as a spring). The hold-down is releasable, to permit the dockboard to be rotated or pivoted between positions.

The hold-down comprises a fluid-containing hyd raulic cylinder assembly having a piston element reciprocating within a cylinder element. This piston element is fluid-tight within the cylinder so that fluid may not bypass it. Fluid-filled chambers on either side of the piston head are formed by the assembly, these being a stem chamber around the piston stem and a base chamber on the opposite side of the piston head. The base chamber of the cylinder is constructed such that the volume of fluid contained therein when the cylinder assembly is fully expanded is greater than the volume of fluid contained in the stem chamber when the cylinder assembly is fully contracted. A conduit connecting the stem and base chambers enables fluid to move between the chambers, thereby permitting expansion and contraction of the cylinder assembly as the dockboard is respectively raised and lowered.A valve means in the conduit controls this fluid movement. There is provided means for mechanically opening the valve means to permit the dockboard to be raised by the dockboard-raising means with concomitant expansion of the cylinder assembly. When such mechanical opening means is inoperative, the valve means is operative (1) to close if there is a greater fluid pressure in the stem chamberthan in the base chamber (thereby locking the device against expansion and preventing the spring from raising the dockboard), and (2) to open if there is a greater fluid pressure in the base chamber than in the stem chamber (thereby permitting the cylinder assembly to contract when a weight acts on the raised dockboard, whereby the dockboard is lowered).

With reference to Figures 6 to 10 of the drawings, the hold-down device 1 is shown mounted in a dockleveller 2. The dockleveller 2 is mounted in a recessed pit of a loading bay 3 such that in the horizontal cross-traffic position, shown in Figure 6, the dockleveller2 is flush with the level of the loading bay platform 3. The dockleveller 2 includes a dockboard 4 hinged at its inner end to a base frame 5. The base frame 5 includes horizontal members 6 and vertical members 7 operable to support the dockboard 4.

Mechanical dock-raising means 8 are shown intermediate the dockboard 4 and the base frame 5.

In the embodiment shown herein the raising means comprise a spring-actuated lever-link mechanism.

Lever-link employing raising means are common to docklevellers, for instance, a lever-link mechanism powered by a hydraulic cylinder is disclosed in U.S.

Patent No. 3,694,840. The lever-link device does not form part of the present invention and is therefore only briefly described herein.

The dock-raising means include parallel spaced link arms 9 pivoted at their outer ends to the base frame 5 forming pivot points 20. The inner end of each link arm 9 is pivoted to a triangular lever arm 10 at a pivot point 21. Lever arm 10 comprises two para llel spaced plates linked together at their vertices with pins, forming pivot points 21, 22 and 23.

A pair of parallel spaced deck lift arms 11 are provided, the outer ends of which are pivotally attached to the base frame 5 at pivot points 24. The triangular lever arms 10 are linked to the lift arms 11 at pivot points 23. the inner ends of the lift arms 11 are pivotally attached to a second pair of parallel spaced triangular lever arms 12 at pivot points 25. The second lever arms 12 are linked at their vertices to form pivot points 25,26 and 27. Parallel spaced sleeve members 13 in rigid attachment to the dockboard, are linked to the lever arm 12 at pivot points 27.

The dock-raising means 8 further include mechanical lift means, such as tension springs, attached intermediate the base frame and the lever-link mechanism. The present embodiment employs at least two such springs. A main lift spring 14 is provided intermediate pivot point 26 on the second triangular lever arms 12 and the hinge 17 atthe rear end of the dockboard 4. The tension of this spring 14 is adjusted so as to provide to the dockboard 4 the majority of the lift force necessary to overcome the gravitational force acting on the dockboard when in a horizontal cross-traffic position. A booster spring 15 is attached between the vertical member 7 of the base frame 5 and pivot point 22 of the lever arms 10.

The tension in the booster spring 15 is sufficient to provide an upward lift force on the dockboard 4 operable to raise the dockboard 4 to the fully raised position shown in Figure 7.

It should be obvious at this point, that the mechanical dock-raising means 8 impart a continuous upward force to the dockboard 4 and in order to lock the dockboard at the cross-traffic or loading positions (Figures 6 and 8) the hold-down 1 is necessary.

The hold-down is mountable intermediate the dockboard 4 and the base frame 5. As shown in Figures 6 and 9, the hold-down is pivotally mounted intermediate pivot points 22 and 28 on the lever-link mechanism. Attachment points are not critical to the function of the hold-down, and one skilled in the dockleveller industry will realize alternate attachment sites.

Figures 6-9 additionally illustrate a lip 16 hinged at the front end of the dockboard 4. Lip-raising means (not shown) are operational to pivot the lip from a vertical hanging position to a horizontal position respectively as the dockboard is pivoted from a horizontal to a raised position. The lip-raising means may be as described in U.S. Patent No. 3,685,076.

It should be realized that the hold-down of the present invention, although disclosed in combination with the spring-actuated lever-link mechanism, could be adapted into other mechanically operated docklevellers.

The hold-down 1 is seen to include a fluid-filled hydraulic cylinder assembly 18 which expands and retracts respectively in response to the dockboard 4 being raised and lowered. Valve means 19 are provided to control the fluid movement within the cylinder assembly 18. Provided the valve means 19 are closed, the cylinder assembly 18 is locked against expansion and thus the dockboard 4 remains in a given position, in opposition to the upward force imparted bythe booster spring 15.

Referring to Figures 11 and 12, the hydraulic cylinder assembly 18 has a base end 29 and a stem end 30. A piston element 31 is provided, having a piston head 32 which reciprocates in a cylinder element 33.

The piston head 32 has a fluid-tight seal 34 with the cylinder element 33, forming a fluid-filled base chamber 35 between the piston head 32 and the base end 29. Fluid-tight seals 36 and 37 are provided between the piston element 31 and the cylinder element 33, thereby forming a fluid-filled stem chamber 38 intermediate the piston head 32 and stem end 30.

The stem and base chambers 38 and 35 are interconnected by a conduit 45. Valve means 19 in the conduit 45 is provided to control the fluid movement between these two chambers.

The base chamber 35 is constructed such that the volume of fluid contained in the base chamber 35 when the cylinder assembly 18 is fully expanded is greater than the volume of fluid contained in stem chamber 38 when the cylinder assembly 18 is fully contracted. This excess fluid volume assures that, on contraction of the cylinder assembly 18, the stem chamber 38 is completely fluid-filled. Anything less than complete filling would result in the inclusion of air into the stem chamber 38, which inclusion destroy the locking function of the hold-down. The reasoning for the inclusion of this excess fluid volume in the base chamber will be apparent once the operation of the system has been described.

This excess volume is provided by including a reservoir tank 39 as part of the cylinder element 33.

Preferably, an inner cylinder 33a is provided, which inner cylinder 33a is open at the base end 29. This open end is housed within a larger diameter reser voirtank39. The inner cylinder 33a forms a first chamber33b between the open end and the piston head 32, and the stem chamber 38 between the piston head 32 and the stem end 30. A port 40 is formed between the inner cylinder 33a and the reservoir tank 39 to permit unrestricted fluid movement therebetween. The reservoir tank or housing 39 is positioned and sealed around a substantial portion of the inner cylinder 33a. A reservoir chamber 41 is thus formed between the outer walls of the inner cylinder 33a and the inner walls of the reservoir tank 39. The reservoir chamber 41, port 40 and first chamber 33b connected in series form the base chamber 35.

The chambers 35 and 38 are filled with a fluid such as a hydraulic oil. The fluid is admitted through a port 42 in the reservoir tank 39, which port is then sealed with plug 43. While it is conceivable that the fluid flow between the stem and base chambers 39, 35 could be achieved by a siphon action, preferably a compressible body 44, such as an air cushion or a spring, is contained in the reservoir tank, the function of which will be later described.

Cylinder rod weldments 46 and 47 are rigidly attached to the ends of the cylinder assembly 18 to facilitate attachment in the dockleveller. As shown in Figure 10, rod weldment 46 rigid with the piston element is linked to the lever arms 10 with pivot pin 22. Rod weldment 47, rigid with the cylinder ele ment, 33, is linked to the link arms 9 with pivot pin 28. In this manner of attachment, the cylinder assembly 18 expands and retracts respectively in response to the dockboard being raised or lowered.

It will be realized that the cylinder assembly 18 could be inverted without affecting the intended function of the hold-down.

With reference to Figures 11 and 12, valve means 19 are shown mounted below the hydraulic cylinder assembly 18. The valve means includes a valve body 48 having a cylindrical bore 49 therethrough. The bore 49 is sealed at one end with plug 50, and at the other end with a plunger element 51.

A narrow seating portion 52 is provided in the bore 49, forming first and second valve chambers 53 and 54 on either side thereof. Conduit 45 leading from the reservoir chamber communicates with the second valve chamber 54 through a port 55. The first valve chamber 53 communicates with the stem chamber through port 56 which is threaded into the cylinder element 33.

In order to control the fluid movement th rough the valve body 48, a check ball 57 is contained in the first valve chamber 53, which check ball 57 is seatable on the narrow seating portion 52 to restrict fluid movement between the two chambers 38 and 41. The check ball 57 is sized to provide a tolerance between the check ball 57 and the inner walls of the first valve chamber 53. In this manner, fluids can bypass the check ball 57 when it is in an unseated position.

Mechanical opening means are provided in the valve means 19 to unseat the check ball 57, thereby opening the valve means to permit the cylinder assembly to be expanded. The mechanical opening means includethe plunger element 51 translatable in the second valve chamber 54. The plunger element 51 has a rod 58 having an inner end 59 in contact with the check ball 57. The outer end 60 of the plunger element 51 is in contact with a control lever 61. The control lever 61 has an upper end 62 pivotally connected to the valve body 48. The lower end 63 of the control lever is attached to a cable 64 (not shown). The opposite end of the cable 64 is attached to a pull ring 65 (not shown) which protrudes through the dockboard 4.

The valve means can thus be opened mechanically by pulling on the pull ring 65. This provides a sufficiently large manual force to the control lever 61 to push the plunger element 51 along the second valve chamber 54 and thereby unseat the check ball 57.

The check ball 57 is preferably spring-loaded to facilitate the seating and unseating of the check ball 57. A compression spring 66 is contained in the first valve chamber 53 compressed between the plug 50 and the check ball 57. This spring 53 exerts a seating force on the check ball 57 and keeps the check ball 57 in alignment with the seating portion 52. This seating force must be overcome in order to unseat the check ball 57. This is accomplished by either pulling on pull ring 65 as described above, or by generating a fluid pressure in the base chamber sufficiently greate than the fluid pressure in the stem chamber.

A plunger return spring 67 is compressed between the plunger element 51 and the narrow seating portion 52. When the plunger element 51 is pushed along the second valve chamber 54, the spring 67 is compressed. Thus when the pull ring 65 is released,the spring 67 is operable to return the plunger element 51 to the position shown in Figure 12.

The valve means 19 as described above are operativeto remain closed if the fluid pressure in the stem chamber is sufficiently greater than the fluid pressure in the reservoir chamber, provided the mechan ical opening means is inoperative.

For safety purposes, a pressure release means 70 can be provided intermediate the stem and base chambers 38 and 35 as shown in Figure 12. The pressure release means constitute a normally closed safety valve functional to open only if the fluid pressure in the stem chamber 38 is greater than a normal operating pressure. By normal operating pressure is meant the fluid pressure generated in the stem chamber as the upward force is imparted on the dockboard by the dock-raising means 8.

Pressures greater than the normal operating pressure may arise from a number of sources. The dockboard in the declined or elevated loading positions essentially rests on the vehicle platform. Any upward vertical movement of the vehicle platform, as is permitted by the vehicle springs, is transmitted to the dockboard and thus into the hold-down. A sufficiently great upward force on the dockboard, in the absence of the pressure release means 70, could cause considerable damage to the hold-down. Additionally, if the pull ring 65 is released before the dockboard is raised to the fully raised position, the valve means immediately close. The momentum of the still moving dockboard, afterthe hold-down is locked, could be sufficient to damage the unit.

The safety valve includes a port 71 through the piston head 32, connecting the stem and base chambers 38 and 35. A check ball 72 is seatable on seat 73, restricting fluid movement between the two chambers when seated. A compressible spring 74 is iocated in the port 71 and is operative to maintain the check ball 72 in a seated position unless the fluid pressure in the stem chamber rises above a normal operating pressure. Once this critical pressure is reached, the compressible spring is compressed, permitting the fluid to move from the stem chamber 38 to the base chamber 35, thereby expanding the cylinder assembly.

Figures 1-5 and 6-9 are illustrative of the operation of the hold-down device 1 in combination with the dockleveller 2.

In the horizontal cross-traffic position shown in Figures 1 and 6, rotation of the dockboard 4 is locked against the continuous upward force imparted by the mechanical dock-raising means 8. More particularly the upward force exerted by the booster spring 15 is attempting to open cylinder assembly 18 and causes the fluid pressure in the stem chamber 38 to be greater than the fluid pressure in the base chamber 35. This fluid pressure in the stem chamber 38 keeps the valve means closed, thereby preventing the expansion of the cylinder assembly.

To raise the dockboard 4, the operator pulls on pull ring 65 which in turn pivots the control lever 61 pushing the plunger element 51 into the valve body 48 to unseat the check ball 57. As the dock-raising means 8 expands the cylinder assembly 18 the fluid moves from the stem chamber to the reservoir chamber as illustrated in Figure 2. In this manner, the mechanical opening means is operative to open the valve means 19 permitting the cylinderassemblyto be expanded as the dock-raising means 8 raises the dockboard 4.

Once the dockboard is raised to the fully raised position shown in Figures 3 and 7, the operator releases the pull ring 65. The plunger return spring 67 and the compression spring 66 then reseat the check ball 57.

To lower the dockboard 4to a loading position, (Figures 5, 8 and 9) level with the vehicle platform (not shown), the operator walks onto the dockboard 4. This man weight causes the compressible body 44 in the reservoir chamber 41 to be compressed, forming a fluid pressure in the reservoir chamber 41 greaterthan that pressure in the stem chamber 38 by an amount sufficient to unseat the check ball. In response to the downward force on the dockboard, fluid movement is directed from the reservoir chamber41 to the stem chamber38 permitting the cylinder assembly to contract, as shown schematically in Figure 4. In addition to opening the check valve, the pressure generated in the reservoir chamber 41 by the compressible body 44 increases the flow rate between the reservoir and stem chambers 39, 41.This in turn enables the cylinder assembly, and thus the dockboard to be rapidly moved between positions and subsequently locked.

Once the lip member 16 of the dockboard contacts the vehicle platform the fluid pressures in the reservoir and stem chambers 41 and 38 are approximately equal and fluid movement therebetween is stopped. The operator then walks off the dockboard.

The dock-raising means 8 immediately pressurize the fluids in the stem chamber 38 which pressure is sufficient to close the valve means 19 and thereby lock the dockboard 4 at this level.

After the vehicle has driven away from the loading bay 3, the operator again walks onto the dockboard.

This man weight, as described above, opens the valve means 19, permitting the cylinder to contract as the dockboard is lowered to the cross-traffic position. The operator then walks off the dockboard; the dock-raising means 8 pressurize the stem chamber fluids to close the valve means and lock the dockboard in this position.

Claims (8)

1. A hold-down device for use in a dockleveller wherein a dockboard is pivotally mounted at its inner end to a base frame for rotation about said pivot between cross-traffic, fully raised and declined positions and wherein mechanical dockboard raising means are provided to rotate the dockboard upwardly from the declined and cross-traffic positions to the fully raised position, said hold-down device comprising a hydraulic cylinder assembly having a stem end and a base end and comprising a piston element, having a piston head, and a cylinder element within which the piston element reciprocates, said assembly being mountable so that one of the elements may be interconnected with the dockboard and the other element may be interconnected with the base frame, whereby the cylinder assembly may expand and contract respectively in response to the dockboard being raised and lowered, said cylinder assembly having a fluid-filled stem chamber and a fluid-filled base chamber, the volume of fluid in the base chamber when the cylinder assembly is fully expanded being greater than the volume of fluid in the stem chamber when the cylinder assembly is fully contracted, said piston head being arranged to prevent fluid movement between the chambers, conduit means interconnecting the stem and base chambers, valve means controlling fluid movement through the conduit means between the chambers, and first means for mechanically opening the valve means to permit the cylinder assembly to be expanded as the dock-raising means raises the dockboard, said valve means being operative to close if there is a greater fluid pressure in the stem chamber than in the base chamber and the first means is inoperative, thereby preventing expansion of the cylinder assembly, and said valve means being operative to open if there is a greater fluid pressure in the base chamber than in the stem chamber, thereby permitting the cylinder assembly to contract

2. A hold-down device as claimed in claim 1, wherein the base chamber comprises a fluid-filled first chamber, and a fluid-filled reservoir chamber, said reservoir chamber being in fluid communication with the first chamber and there is provided a compressible fluid body in the reservoirchamber.

3. A hold-down device as claimed in claim 2, wherein the cylinder element comprises an inner cylinder and a reservoir tank, the inner cylinder being open at one end which open end is housed and sealed within the reservoir tank, the piston element reciprocates within the inner cylinder, the first fluid-filled chamber is formed between the piston head and the open end of the inner cylinder, the stem chamber is formed between the piston head and the stem end of the cylinder assembly and the reservoir chamber is formed between the walls of the inner cylinder and reservoirtank, a port is provided between the reservoir and first chambers permitting fluid communication therebetween, and the reservoir chamber, port and first chamber together form the base chamber.

4. A hold-down device as claimed in claim 3, wherein the valve means comprises a valve body positioned in the conduit means and having a valve seat, and a check ball seatable on the seat to control fluid movement through the valve body between the reservoir and stem chambers, the first means being manually operable to unseat the check ball and thereby permit the cylinderto be expanded as the dock-raising means raises the dockboard, and said check ball being movable between an opened and closed position respectively in response to the greater of the fluid pressures bearing thereon from the reservoir and the stem chambers, provided the first means is inoperative.

5. A hold-down device as claimed in claim 4, wherein the valve means further comprises a first spring in the valve body operative to maintain a seating force on the check ball, the first means comprising a manually operative plunger in contact with the check ball, said plunger being translatable from a rest position in the valve body to unseat the check ball when a sufficiently large manual force is applied to the plunger, and a second spring in the valve body operative to return the plunger to the rest position in the valve body when the manual force is released, thereby allowing the check ball to reseat.

6. A hold-down device as claimed in claim 5, wherein the pressure release means are provided intermediate the stem and base chambers, said pressure release means comprising a port interconnecting the stem and base chambers, normally closed valve means controlling the fluid movement through the port between the chambers, a compressible spring in contact with the valve means, said compressible spring being operative to maintain the valve means in a closed position provided the fluid pressure in the stem chamber is at a normal operating pressure, and said compressible spring being compressed when the fluid pressure in the stem chamber is greater than a normal operating pressure, thereby permitting the valve to open and the cylinder assembly to expand.

7. A hold-down device substantially as hereinbefore described with reference to the accompanying drawings.

8. A dockleveller incorporating a hold-down device as claimed in any one of claims 1 to 7.