GB2127913A - Electromagnetic clutch for transport refrigeration units - Google Patents

Abstract

An electromagnetic clutch for coupling two substantially axially aligned shafts (12, 20) includes an armature plate (36) rigidly attached to an armature drive plate (40) which, in turn, is connected to a flywheel (14) and located relatively thereon by means of drive pins (46) which are easily slidably received in resilient bushings (44) in bores (42) of the flywheel. The armature plate is engageable with a pulley (18) rigidly attached to the shaft (20) and having a portion including electromagnetic means and through which the flux (60) flows, upon excitation of the latter, to engage the clutch. The armature plate is returned by springs (70) and the working clearance between that plate and the pulley is adjustable by bolts (65). <IMAGE>

Description

SPECIFICATION Electromagnetic clutch particularly suited for transport refrigeration units This invention pertains to the art of electromagnetic clutches, and particularly to a clutch for coupling an internal combustion engine with a refrigerant compressor where the shafts of the engine and compressor are in substantial axial alignment.

One common way for coupling an internal combustion engine of a transport refrigeration unit to the in-line refrigerant compressor is to use a centrifugal clutch as is disclosed, for example, in U.S. Patent 3,941,012. Because of the severity of service under which such clutches are required to operate, the typical current production centrifugal clutch of the assignee of this application has a life expectancy of somewhere between 3,000 to 5,000 hours. One reason for this relatively limited life expectancy is because of the high stresses imposed on the clutch components due to torque pulses from the internal combustion engine. Centrifugal clutches also require bearings which constitute a service problem and, since the clutch is speed responsive, it cannot be independently decoupled when the speed of the engine is above that which effects the coupling.

Electromagnetic clutches for coupling an engine to drive a refrigeration compressor are known, for example, as disclosed in U.S.

Patent 4,187,939. There, an automotive air conditioning compressor is adapted to be coupled to a belt-driven pulley by means of an electromagnetic clutch arrangement. In that arrangement, a pulley which is belt driven from the engine is adapted to be coupled to rotate the compressor shaft through an electromagnetic clutch. In that arrangement, torque pulses or spikes from the engine are absorbed by the belt. The belt-driven pulley rides on bearings which, in turn, are carried by the compressor shaft sleeve and, since everything is piloted off the compressor shaft, there basically are not any problems of lack of concentricity of the facing clutch plates.

It will be appreciated, of course, that two coupling faces which lie in a plane perpendicular to the axis of the shafts to be coupled should be concentric or else the faces will constantly be moving relative to each other while they are coupled. In a transport refrigeration unit, half of the clutch pilots off the engine shaft, while the other half of the clutch pilots off the compressor shaft. Thus, the concentricity of the two clutch faces will only be as good as the alignment between the two shafts plus the build-in tolerances of the various pieces which lead to the two clutch faces.

It is the aim of my invention to provide an electromagnetic clutch for such an environ ment in which bearings need not be employed and in which a slight amount of misalignment of the two shafts can be accommodated and in which torsional dampening is provided to accommodate torque pulses from the engine.

In accordance with the invention, an electromagnetic clutch arrangement is provided for coupling an engine flywheel, which is rigidly fixed to the rotating shaft of the engine, to drive the compressor which is powered through the compressor shaft which is in substantial axial alignment with the engine shaft. The arrangement includes a rotatable annular electromagnetic means which is rigidly fixed directly to the compressor shaft, an annular armature facing the electromagnetic means and fixedly attached to an annular armature drive plate.Means connecting the armature drive plate to the flywheel comprise elastomeric bushings in a number of bores circumferentially spaced about the flywheel with an equal number of drive pins fixed to the armature drive plate and received in the bushings in an easily slidable fit so that upon excitation of the coil the armature and drive plate can freely move into contact with the electromagnetic means without the bushings retarding the movement of the drive pins along with the drive plate.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of the drive system parts of a transport refrigeration unit; Figure 2 is a sectional view of an electromagnetic clutch arrangement according to the invention for coupling an engine to a compressor; and Figure 3 is a face view of a fragmentary part of the armature and armature drive plate.

Referring to Fig. 1, the power system parts for a transport refrigeration unit are schematically shown and include an internal combustion engine such as a diesel engine 10, which through its crankshaft 1 2 drives a flywheel 1 4 which carries one-half 1 6 of an electric clutch, with the other half of the clutch comprising a pulley 1 8 fixed to the shaft 20 of the compressor 22. In the particular embodiment shown in Fig. 1, an electric motor 24 is included to provide power through pulley 26 and belt 28 to the compressor 22 when the internal combustion engine 10 is not operating and the clutch is decoupled. It is noted that the invention is independent of whether or not the half 1 8 of the clutch is in the form of a pulley or not.

Referring to Fig. 2, the flywheel 14 rigidly attached to the engine crankshaft 1 2 is housed within a flywheel housing 30 to which is attached, in a piloted relationship off surface 32, the compressor seal plate 34. By carefully dimensioning the parts of the compressor from the point 32 to the location of the compressor shaft 20 it is intended that the axial alignment of the engine shaft 1 2 and compressor shaft 20 be as close as is reasonably possible.

The armature plate 36 is rigidly attached as by screws 38 to the armature drive plate 40 which, in turn, is connected to the flywheel 14 in a manner to provide torsional damping.

The connecting means includes the provision of a number of bores 42 spaced circumferentially about the flywheel, and into which are press-fitted resilient bushings 44. The armature drive plate 40 has a number of drive pins 46 press-fitted into it with these pins projecting into and being received in the bushings 44 in an easily slidable fit so that movement of the drive pins relative to the bushings is unrestricted. Thus, the drive to the armature 36, which serves as one face of the clutch, is from engine crank 12, flywheel 14, bushings 44, drive pins 46, drive plate 40, to the armature 36.

The other half of the clutch is included in the pulley 1 8 which is rigidly fixed directly to the compressor shaft 20 as by keying or other means. The pulley is provided with an annular recess 48 within which the field assembly 50 is located. This field assembly includes the annular coil 52 and is attached to the compressor seal plate by bolts 54.

The face of the pulley serving as one-half of the clutch is provided with a series of circumferentially extending, disconnected slots 56 which face correspondingly located grooves 58 in the armature 36 (Fig. 3) so that when the coil 52 is excited, the flux path will be as indicated by the broken line 60 with two passes through the armature plate 36. The magnetic force developed by the flux draws the armature plate 36 and armature drive plate 40 to the pulley 1 8 through the gap 62 and the clutch is then in the coupled condition. The movement of the armature drive plate 40 toward the pulley is unrestrained with respect to any snubbing or retarding action between the bushings 44 and pins 46.

While not considered absolutely necessary, it is considered preferably that means be provided to bias the armature drive plate away from the pulley, and this is accomplished in the illustrated embodiment by the provision of bolts 64 turned into the flywheel 14 through oversize holes 66 in the armature drive plate, and with a washer 68 and compression spring 70. The oversize holes provide clearance to prevent bolts 64 from coming into contact with armature drive plate 40. This relative movement between the armature drive plate 40 and bolts 64 occurs as the bushings 44 yield to absorb torsional spikes particularly during clutch engagement. Thus, the oversize holes prevent the bolts from influencing the radial location of the armature drive plate.

The clearance between the armature plate 36 and pulley clutch face should be maintained within certain limits, ususally between 0.03 to 0.05 inches (0.76 mm to 1.27 mm).

This adjustment can usually be made by adjusting the distance between the drive plate 40 and the flywheel 14 by means of bolts 65 and lock nuts 67 turned into tapped opening 69 of the drive plate. In the case of a stack-up of manufacturing tolerances increasing the air gap to the point where the adjustment available from the bolts and lock nuts is not sufficient, shims 71 can be placed between the armature plate 36 and the armature drive plate 40 can relieve the problem. Such shims may also be necessary after the pulley is refaced to clean up grooves worn into the pulley face due to wear.

The operation of the clutch should be apparent from the foregoing. When the coil 52 is excited, the flow of flux through that part of the pulley which constitutes the rotatable annular electromagnetic means generates the force to draw the armature plate 36 and armature drive plate 40 into coupled relation.

When the excitation of the coil 52 is removed, the compression springs 70 push the drive plate 40 back against the flywheel and thus carries the armature plate 36 to a decoupled position.

While every effort is made to obtain precise axial alignment of the two shafts on the opposite sides of the clutch, in practice the alignment is typically imperfect. Since the one side of the clutch is piloted off the engine shaft 1 2 and the other off the compressor shaft 20, there may not be perfect concentricity of the two clutch faces. With my arrangement, any slight lack of concentricity can be accommodated sufficiently by the yielding actions of the rubber bushings 44. These rubber bushings also serve to accommodate any torque pulses or spikes originating from the engine.

It will be noted that the arrangement according to my invention is bearingless; bearings in electric clutches typically providing a prime service problem. Also the arrangement of my invention does not include any flat or leaf springs which are usually included in automotive air conditioning electric clutches of the character disclosed in the one noted patent.

Claims (6)

1. An electromagnetic clutch arrangement for coupling a flywheel on a shaft of a prime mover to drive a load powered through a load shaft which is in substantial axial alignment with the prime mover shaft, comprising annular electromagnetic coupling means affixed to the load shaft for rotation therewith; an armature plate facing said electromagnetic means and affixed to an armature drive plate; and means connecting said armature drive plate to said flywheel, comprising elastomeric bushings in bores formed axially in said flywheel and spaced circumferentially about the axis thereof, a plurality of drive pins disposed on said armature drive plate and projecting therefrom into the respective bushings so as to form therewith an easily slidable fit enabling the armature plate together with the armature drive plate to move freely into coupling engagement with said electromagnetic coupling means upon excitation thereof, said bushings permitting the drive pins to move therein unretardedly and cooperating with the pins to provide torsional damping between the armature drive plate and the flywheel.

2. An electromagnetic clutch arrangement according to claim 1, wherein said armature drive plate is biased toward the flywheel.

3. An electromagnetic clutch arrangement according to claim 1 or 2, wherein said prime mover comprises an internal combustion engine, and said load comprises a compressor.

4. An electromagnetic clutch arrangement according to claim 3, wherein said compressor is a refrigeration compressor.

5. A transport refrigeration drive system for coupling a driving engine to a driven refrigeration compressor, comprising a driving internal combustion engine having a flywheel housing with a flywheel therein driven by the engine crankshaft, said flywheel having a number of bores formed therein and spaced circumferentially thereabout; a compressor including a housing attached to said flywheel housing in a piloting positioning relationship to place the compressor crankshaft in substantially direct axial alignment with said engine crankshaft; rotatable annular electromagnetic means rigidly fixed directly to said compressor shaft; an annular armature facing said electromagnetic means and fixedly attached to an annular armature drive plate; elastomeric bushing means in each said bore; and drive pin means fixed to said armature drive plate and received in said bushing means in an easily slidable fit so that, upon excitation of said electromagnetic means, said armature and drive plate can freely move into dontact with said electromagnetic means without said bushings retarding movement of said drive pins.

6. A system according to claim 5, including means biasing said armature drive plate towards said flywheel and effective upon deexcitation of said electromagnetic means to assure decoupling.