US2240966A

US2240966A - Piston

Info

Publication number: US2240966A
Application number: US299050A
Authority: US
Inventors: William M Venner; Jr Percy L Bowser
Original assignee: STERLING CORP
Current assignee: STERLING Corp
Priority date: 1939-10-12
Filing date: 1939-10-12
Publication date: 1941-05-06
Anticipated expiration: 1958-05-06

Description

Patented May 6, 1941 William M. Venner, Richmond Heights, and Percy L. Bowser, Jr., University City, Mo., assignors t The Sterling Corporation, St. Louis, Mo., a corporation of Delaware Application October 12, 1939, Serial No. 299,050 11 Claims. (Cl. 3 09-13) This invention pertains to pistons particularly for internal combustion engines, and more particularly to pistons of the trunk type made of light metal, such as aluminum or magnesium or alloys thereof.

. A major problem in the construction of all light metal pistons has been to provide accurate compensation for the high rate of thermal expansion of the piston metal, particularly aluminum and its alloys. This, of course, is due to the fact that the rate of thermal expansion, which is measured bythe coefcient of expansion, is about twice as great for aluminum as it is for cast iron, and, since an aluminum piston must ordinarily work within a cast iron cylinder, this excess expansion of the piston would require an excessive clearance between piston and cylinder if no compensation for expansion could be made.

The amount of compensation required is `somewhat diierent at the Vupper .or head end of the piston than it is at the lower end. This is chiefly on account of the fact that there is a fallingtemperature gradient from the head end to the lower end since the piston head is heated ,by the combustion gases, and this heat ows downwardly through the piston to the cylinder walls where it is delivered to the cooling water surrounding those walls.

In the all-aluminum commercial pistons now skirt in the regionsvof the cylinder-bearing portions or thrust'faces, and the skirt is relieved in the regions of the pin bosses or bearings, while the head diameter is reduced. The piston is so constructed that the thrust faces will fit with the desired clearance when the piston is cold, As the piston heats up, while the `head will expand uniformly, the head expansion-is transmitted to the skirt in the regions of the bosses, where there is relief, and thethrust faces are drawn in. While this does, to an extent, compensate for expansion the thrust faces bear heavily at their ends or corners, particularly at the top. This bearing is the result not only of the outward movement along the wrist-pin axis, but also cordal movement along the connectors between vthe head bosses and skirt. Commercial pistons are therefore further relieved at these corners by what is known a's a keystone relief, or by cam grinding of the skirt to an elliptical form with the lminor axis of the ellipse along the wrist-pin axis. is obviated in a strut piston in which the cordal members are of a material having a .rate of exin the direction along the pin axis. Strut pistons are, therefore, also cam ground. However, whether keystone reliefs or cam grinding is em ployed, this will not only fail to secure a uniform bearing face when the piston is cold, but will also tend to cause side slap.

4'Ihe purpose of this invention, generally stated,v is to provide a piston which can be constructed practically entirely of light metal and which has its structure so organized as 'to compensate to a certain degree for the excessive rate of expansion r of the piston metal, with the addition that near the upper part of the bearing portion of the skirt additional control of such expansion is introduced by adding one or more control elements made of a material having a lower rate of thermal expansion than the piston material. In accordance with an embodiment of this invention, such control element is so arranged and positioned as to control the curvature of the upper part of the bearing portion of the skirt.

The further object of this invention is to provide a piston structure so controlled as to its expansion that the bearing portion may be finished to substantially a true circular shape and still maintain a good bearing fit in the cylinder at all operating temperatures.

Further`- objects will appear from the following description, taken in connection with the accomv panying drawing, in which:

Figure 1 is a side view taken along the wrist pinv axis of a piston embodyingthis invention;

Figure 2 is a side view of the piston of Figure 1 showing another embodiment; V

Figure 3 is a longitudinal section taken in two planes, indicated by the lines 3-3 of Figure 5; Figure 4 is a side view similar` to Figure 2 of another piston embodying this invention;

While the cordal expansion l Figure 5 is a transverse section taken in two planes, indicated by lines 5--5 of Figure 3, and; Figure 6 is a perspective viewof one form of control element.

Generally statedand in accordance with this invention, a light metal piston comprising a head,

. a skirt and wrist-pin bearings connected with the connectors between the head and the skirt. An

element having a rate of yexpansion lower than 'the cylinder-engaging portion is arranged to ex\ the cylinder-engaging portion, as by casting, so as toflex the cylinder-engaging portion inwardly in the regions of the vbearings thermostatically.

Referring now to the drawing, the piston shown is cast substantially entirely of aluminum alloy and has a head I provided with the usual packing ring grooves 2. Wrist pin bearings 3 are secured to the head I by downwardly extending connectors 4, reinforced by ribs 5, The piston skirt,

indicated generally at 6, is attached to the head and between the cylinder-engaging portions 1, are relieved, as indicated at 8. This relief is provided preferably by slightly indenting this portion of the casting so that its maximum diameter is sufliciently less than the cylinder bore to provide ample clearance at all temperatures.

In order to provide for proper operation of the piston in the cylinder, that portion of the head I which carries the packing rings in the grooves 2 is machined to a relatively small diameter, providing suicient clearance so that this head portion will never contact with the cylinder walls chined to a small enough diameter to provide ample clearance at all times. The cylinderengaging portions 1, however, are finished to such a diameter as to allow just sufficient clearance to provide a smooth sliding iit under proper conditions of lubrication. These portions function to guide the piston and 'support the lateral thrusts. due to angularity of the connecting rod. In accordance with the embodimentshown in Figures 1, 2, and 3, the cylinder-engaging portions 1 are separated from the head I by-horizontal slots or gaps 9 and from the bearings 3 and connectors 4 by downwardly extending slots I0. In the particular embodiment illustrated, the slots I0 extend downwardly and turn laterally, as indicated at I I, so as to form at the upper lateral corners of the cylinder-engaging portions 1 laterally extending wings I2. The limiting edge of the relieved area 8 may beextended upwardly at an angle, as indicated at I3, so as to carry this relieved area around the ends of-the lateral slots II.

The relieved area 8 may be carried clear to the lower edge of the skirt on both sides, as shown in Figure 1. 'It may, however, be desirable for some purposes to limit the downward extent of this area and provide at the lower end of the piston a circumferential rim portion I4, arranged to bear against a cylinder wall. Such a rim por-l tion may be provided at only one side of the piston, as shown in full lines in Figure 2,v or such a portion may be provided on both sides, as indicated in dotted lines in that ligure. 'I'hese variationsvare determined in most instances by the vrequirements of particular engines.

In accordance with the illustrated embodiments of this invention, a control element I5 is provided having a lower rate of thermal expansion than the piston metal. This element may be constructed of steel, which is cheap and easily worked, and having a coefiicient of expansion less than that of the piston metal, or it may beof other suitable material having thefproperl c0- eicient of thermal expansion. In the embodiment illustrated, it is provided in the form of a flat strip of arcuate shape and provided `on-its outer circumference with a series of recesses I6, adapted to interlock withthe piston metal during casting. In the embodiment shown, the recesses I6 are formed by simply drilling or punching round holes close enough to the outer rim of the element I5 so as to overlap said rim, leaving the recesses open at the outside.

One or more elements I5 are applied to the piston by being supported in the mold iny any v suitable manner when the piston is cast, so as to be cast into the piston meta-l in the position shown in Figures 1 to 5, inclusive. As will be seen from Figure 5, the element I5 extends circumferentially around the piston skirt along the inside thereof, and the piston metal flows into the recesses I6 during the casting operation so as to provide a secure interlock between th element I5 and the piston skirt. It will be seen that the element is positioned so as to extend horizontally along the inside of the cylinderengaging portion of the skirt near its top and soas to extend circumferentially in both directions from the middle of the `portion 1 toward the outer top corners and the wings I2.

It will be noted, particularly from Figure 5,

I that the element 'I5 is so related to the cylindereven at the highest operating temperatures. f Similarly, the ends of the bearings 3 are maengaging portion 'I as to provide therewith a bimetallic member extending horizontally along the top part of said portion.' Such a vbi-metallic member may act thermostatically upon increase of temperature, and, since the less expansible member I5 isv on the inside of `the curve, rise of temperature will tend to cause an inward curling of thisbi-metallic part so as to tend to draw radially inwardly the lateral top corners of the l end. It has been found that the control element I5 may be so proportioned relatively to the `piston skirt that this thermostatic action may be adjusted to compensate almost exactly for the excess expansion of the aluminum piston over that of the iron cylinder.

It will be noted that the arrangement of the slot I0 is such as to free the outer -top corners of the cylinder-engaging,portions 1 from the wrist pin bearings and thereby render said corners more radially. flexible. Accordingly, the full benefit of the thermostatic action just described may be obtained at these upper corners.

At the -lower end of the skirt the excess ei:-v pansion of the piston over that of the cylinder is less than at the top. Such excess may be' taken care of by providing the relief 8, which permits the expansion of the wrist pin bearings 3 as transmitted to the lower end of the skirt in the direction along the pin axis, to stretch the lower end of the skirt in that direction and thereby tend to draw inwardly the lower ends of the portions 1.

Generally stated, the operation is as follows,

referring to the drawing: When the piston is,

cold the cylinder-engaging portions, particularly at the top, will engage the cylinder substantially circumferentially so that the piston is supported in such a manner-that no slapping will take place. As the head of the piston heats up, the head expansion is. of course, transmitted to the skirt along the wrist-pin axis, which will normally result in a drawing-in of the'cylinderengaging portions. The bimetallic structure will, however, cause the terminals of the bearing faces at the top to move in so as to maintain the engagement as before and so as to avoid rubbing at the corners. Accordingly, the skirt will maintain its shape circumferentially as the piston heats up and will follow the expansion of the cylinder.

It will be seen, therefore, that inaccordance with this invention, a piston is provided which is substantially entirely of cast light metal. The

excess expansion at the lower end of the'skirt is taken care of by the light metal structure of the piston, while at the upper end of the skirt where the temperature is higher the control I operates to draw in the upper side corners of the cylinder-engaging portions so as to compensate for excess expansion at this point. It has been found that with this construction the portions 1 may be finished to substantially a true circular shape instead of the eliptical shape, which has become almost standard practice for aluminum pistons. When so finished to circular shape, this piston maintains a nearly perfect fit in the cylinder at all operating temperatures. Pistons of this construction have. been fitted with a clearance of .0015 to .002 inch when cold, and have given perfectly satisfactory operation under all conditions of load. y

In the embodiment of Figure 4, the slots IM around to provide the horizontal portions II. This provides a somewhat stiffer skirt at the top, while still freeing the upper corners from the pin bearings or the connectors l, so as to permit those corners to flex. In this embodiment, also, the side edge i3 of the relieved area 8 has been straightened somewhat. This provides a broader cylinder-engaging portion 1.

Such a structure may be desirable under conditions where a stiffer skirt is required. It is believed, also, that in this embodiment the controlling action ofthe member 'I5 may be carried downwardly along the skirt to a greater extent than in the embodiment'of Figures 1 and 2.

It will be understood, of course, that in any of the embodiments any of the arrangements of the relieved area 8, vshown in Figures l and 2, may be employed, according to the requirements of the particular situation. Furthermore, the slots I0 and Il maybe extended more or less so as to control the exibility of the upper corners of the bearing portion, or to provide wings l2 of greater or less extent.

with said cylinder-engaging portion so as to extend circumferentially along and within the upper part thereof toward said side corners, the upper part of said cylinder-engaging portion and said control element'forming a bi-metallic structure thermostatically controlling the expansion of said side corners.

2. A light-metal piston of the character described, having a head, wrist-pin bearings rigidly connected with said head, a skirt connected with said bearings but having a cylinder-engaging portion separated from said head inthe region between said bearings, and said cylinderbearing portion having upper side corners free of said pin bearings so as to be flexible relatively thereto, and a flat control element of a material having a lower rate of thermal expansion than the piston metal, cast into and interlocked with said cylinder-engaging portion so as to extend circumferentially along and within the upper part thereof toward said side corners with its flat face in a plane transverse to the piston axis, the upper part of said cylinder-engaging portion and said control element forming a bi-metallic have been terminated without carrying them l It is obvious that various changes in the de-l t scope of the appended claims without departing from the spirit of this invention, and it is understood the invention is not limited to the specific details shown and described.

The invention having thus been what is claimed is:

l. A light-metal piston of the character described, having a head, wrist-pin bearings rigidly connected with said head, a skirt connected with said bearings but having a cylinderengaging portion separated fromV said headin the region between said bearings, said cylinderengaging portion having upper side corners free of said pin bearings so as to be flexible relatively thereto, and a control element of a material having a lower rate of thermal expansion than the piston metal, cast into and `interlooked described,

with said bearings but having a cylinder-engag- `ing portion separated from said head and terminating circumferentially in the regions between` said bearings to provide upper side corners, and a control element of a material having a'lower rate of thermal expansion than the piston metal, secured to the inside of said cylinder-engaging portion and extending between the upper side corners thereof to control the curvature of said portion thermostatically.

4. A light-metal piston of the character described, having a head, wrist-pin bearings rigidly connected with said head, a skirt connected with said bearings but having a cylinder-en- `gaging portion separated from said head and terminating circumferentially in the region between said bearings to provide upper side curves, and a control element of a `material having a lower rate of thermal expansion than the piston metal, secured to the inside of said cylinder-engaging portion and extending between the upper side corners thereof to control the curvature of said bearing portion thermost-atically, said skirt being finished at its cylinder-engaging portion to a substantially true circular form but being relieved in the region adjacent an end of the wrist-pin axis.

5,. A light-metal piston of the character described, having a head, wrist-pin bearings rigidly connected with said head, a skirt connected with said bearings but having a cylinder-engaging portion separated from said head in the region between said bearings, said cylinder-engaging portion having 'a laterally extending wing scribed, having a head, wrist-pin bearings rigidly scribed, having a head, a skirt, wrist pin bearings connected with said head and said skirt, said skirt having a cylinder-engaging portion whose upper side corners are separated from said head and said bearings so as to be flexible relatively thereto, and a control element of a ma-f terial having a lower rate of thermal expansion Vthan the piston metal, cast into and interlocked with said cylinder-engaging portion so as to extend circumferentially along and within the upper part thereof toward said side corners, the upper part of said cylinder-engaging portion and said control element forming `a bi-metallic structure thermostatically controlling the expansion of said 'side corners.

bearings, connectors between said head and said skirt, said skirt having a cylinder-engaging portion separated from said head circumferentially and separated from said connectors terminally,

8. A light metal piston of the character described, comprising, a head, a. skirt, wrist pin bearings connected with said head and said skirt, said skirt having a cylinder-engaging portion separated from said head and constructed to fiex inwardly in the regions of said bearings, and ari-element having a rate of expansion. vlower than, extending circumferentially inside of and attached to, said cylinder-engaging portion, adapted to flex said portion inwardly in said regions thermostatically.

9. A light metal piston of the character described, comprising, a head, a skirt, wrist pin constructed to iiexinwardly in the regions of said bearings,` and an element having arate ofexpansion lower than said' cylinder-engaging portion, said element extending circumferentially inside of and cast to said portion and terminating therewith, adapted to ex said portion inwardly in said regions thermostatically.

10. A light metal piston of the character described, comprising, a head, a skirt, wrist pin bearings connected with said head and said skirt, said skirt having cylinder-engaging portions separated from said had and constructed to flex inwardly inthe regions of said bearings, and an element extending circumferentially inside of and attached to each of .said cylinder-engaging portions, said element having a rate of expansion lower than that of said portion and adapted to ex said. portion inwardly in said regions thermostatically. I

11. A light metal piston of the character described, comprising, a head, a skirt, wrist pin bearings, connectors between said head and said skirt, said skirt having cylinder-engaging portions separated from said head circumferentially and separated from said connectors terminally, constructed to flex inwardly in the regions of said bearings, and elements having a rate of expansion lower than that of said cylinder-engaging portions, oneof said elements extending circumferentially inside of and cast to each of said portions and terminating therewith, adapted to iiex each said portion inwardly in said regions thermostatically.

WILLIAM M. VENINER.v PERCY L. BOWSER, JR.