Air compressor having condition responsive clutch control

An air compressor has a piston (3) reciprocable to compress air within a cylinder (2), a clutch (16) to transmit drive from a drive input shaft (12) to the piston when compression is required, a piston (27) arranged to be responsive to pressure resulting from the compressor output operable to disengage the clutch (16) and thereby interrupt drive to the compressor piston when the pressure reaches a predetermined value, and a passage (37) to provide a leakage path for the pressure applied to the piston (27) to enable the pressure to decay in a controlled manner when further operation of the piston is required, whereby engagement of the clutch is permitted to occur progressively and shock loadings on the compressor components are minimized.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an air compressor, primarily for use in a vehicle 
air-actuated braking system, of the type having at least one compressor 
element operable by drive means to effect compression of air within a 
space, the compressed air usually being fed, in use, to a reservoir and 
thence on demand to the braking system for actuation of the latter. 
2. Description of the Prior Art 
In FIG. 1 of our co-pending British Patent Application No. 8215732 there is 
described an air compressor in which a compressor element is operable by 
drive means to effect compression of air within a space, the arrangement 
including a clutch for transmitting drive from the drive means to the 
compressor element when compression is required, the clutch being normally 
held in engagement by a spring device and being disengagable by a device 
responsive to a pressure resulting from the compressor output to interrupt 
the drive to the compressor element when said pressure reaches a 
predetermined value. 
With such an arrangement, it is desirable to re-engage the clutch gradually 
after disengagement thereof in order to minimize shock loading of the 
compressor components. In our aforesaid British Application, it is 
proposed to use an unloader valve to hold open an inlet valve of the 
compressor temporarily in order to prevent compression during at least the 
initial stage of clutch re-engagement. However, this can be unduly 
complicated in practice and, in some circumstances, not entirely 
satisfactory in operation. 
BRIEF SUMMARY OF THE INVENTION 
An object of the invention is to provide an air compressor of the aforesaid 
kind incorporating improved means for effecting gradual re-engagement of 
the clutch. 
According to the invention, an air compressor comprises at least one 
compressor element operable by drive means to effect compression of air 
within a space, a clutch for transmitting drive from the drive means to 
the compressor element when compression is required, a pressure device 
responsive to a pressure resulting from the compressor output and operable 
to disengage the clutch when said pressure reaches a predetermined value, 
resulting in an interruption of drive to the compressor element, and means 
providing a leakage path permitting pressure to decay in controlled manner 
from said device whereby clutch re-engagement is permitted to occur 
progressively. 
In one convenient arrangement, said pressure is applied to said device by 
way of a one-way valve, and leakage path then being preferably 
incorporated in said valve, conveniently in the form of a passage through 
a valve member thereof. 
Conveniently, a sleeve surrounds and is spaced from the longitudinal axis 
of a drive input shaft from the drive means, the sleeve containing the 
clutch and being drivingly engaged with a part of the latter. In one 
convenient arrangement, the sleeve is carried by said shaft and rotates 
with the latter. 
The pressure device may conveniently be an annular piston surrounding said 
longitudinal axis and disposed at one side of an axially outwardly 
directed wall of the sleeve and acting on the latter via thrust means, the 
spring device also surrounding said longitudinal axis and acting on the 
opposite side of said sleeve wall in opposition to the action of said 
piston. 
Advantageously, the pressure device may surround the drive input shaft and, 
in such an arrangement, a spring device acting in opposition to the 
pressure device, is advantageously located between said sleeve wall and an 
opposed wall of a radial flange of sais drive input shaft. This enables 
both the sleeve and thrust bearing to rotate continuously with the input 
shaft which means that the bearing is not subjected to accelerations 
during operation of the clutch and, since only the inertia of the clutch 
driven plates has to be overcome, in addition to that of the compressor 
components, the clutch capacity can be relatively small. 
The clutch is conveniently pre-loaded in the clutch engagement direction, 
preferably by an additional spring means which may conveniently act 
between said pressure device and a fixed part of the compressor. 
Such pre-loading is applied to ensure that the clutch engagement force 
remains adequate even when the force applied by the spring device is 
lessened as a result of its extending to compensate for the effect of wear 
of the clutch linings. 
Preferably, said pre-load is applied at the side of the piston opposite to 
that acting on the sleeve. 
The drive input shaft may conveniently be provided with a hollow co-axial 
boss which surrounds an adjacent end portion of a driven shaft of the 
compressor and is supported for rotation on the latter by the intermediary 
of a bearing. This helps to minimize the occurrence of eccentricities 
between the various co-axially mounted components. 
In another convenient arrangement, the drive input shaft from the drive 
means is mounted with respect to a fixed part of the compressor by way of 
a tapered bearing. 
Preferably, the drive input shaft is rotatably supported relative to a 
driven compressor shaft by means of a further tapered bearing. This 
arrangement can be especially useful when the drive input shaft receives 
its drive from the drive means by way of a helical gear drive since it 
reacts the axial component of force applied to the shaft from such a gear 
arrangement. 
Typically, the taper bearings are arranged to be tapered in opposite 
directions in relation to the respective longitudinal axes of the shafts.

DETAILED DESCRIPTION 
Referring to FIG. 1 of the drawings, the compressor illustrated therein is 
primarily intended for use with a vehicle air-actuated braking system, of 
which a part is illustrated diagrammatically. The compressor comprises a 
housing 1 defining a cylinder 2 within which slides a piston 3, coupled by 
a connecting rod 4 to a crank shaft 5 supported in bearings 6 mounted in 
the housing. As indicated by the arrows, air enters the cylinder during 
the induction stroke of the piston via an inlet port 7 and leaves the 
cylinder via an exhaust port 8, the ports being controlled by reed valves, 
in conventional manner. The crank shaft 5 has an axial extension 9, an end 
portion of which carries a bearing bush 10 which is telescopically engaged 
within a hollow end portion 11 of a drive input shaft 12 rotatably 
supported in a bearing 13 mounted in a sub-housing 14 which is secured to 
the housing 1 by bolts 15 and surrounds the outer end portion of the crank 
shaft extension 9. 
The crank shaft is driven from the drive input shaft 12 by a multi-plate 
clutch, indicated generally at 16, which may be dry or oil immersed, and 
which includes a plurality of driving clutch plates 17 mounted, by way of 
splines 18 as shown or by other suitable means, within a hollow clutch 
member 19 which surrounds the clutch and is rotatable with the shaft 12. 
The clutch plates 17 are thus fast for rotation with but axially slidable 
relative to the clutch member 19. The clutch further includes a plurality 
of driven plates 20 inter-leaved with the driving plates 17 and being fast 
for rotation with but axially slidable relative to the crank shaft 
extension 9, by way of splines 21. The clutch plates are retained within 
the clutch member 19 by means of a circlip 22. 
The clutch plates are normally urged into driving engagement with a second 
clutch member 23 formed by a radial flange of the drive input shaft 12, by 
means of a conical disc spring washer assembly 24 acting between the side 
of the clutch member 23 remote from the clutch plates and the opposed 
surface of a radially extending flange 25 of the clutch member 19, this 
clutch member being slidable along the drive input shaft 12 to enable the 
force of the washer assembly to be applied to the clutch plates through 
the circlip 22. The clutch member 23 is drivingly engaged with the splines 
18 of the clutch member 19. 
The sub-housing 14 forms an annular chamber 26 containing an annular piston 
27 slidable therein, the piston acting via a thrust bearing 28 on the 
outer surface of the flange 25 of the clutch member 19. A control valve 29 
is provided within the housing 1 and the chamber 26 is connected by way of 
this valve and a suitable governor valve 30, such as the type shown in 
British Pat. No. 935,567, for example, to a reservoir 31 connected to the 
outlet port 8 of the compressor. The reservoir would normally be connected 
via a line A to one or more brake actuators (not shown) of a conventional 
brake system 50 of a vehicle braking system, in conventional manner. 
The control valve 29 contains a valve chamber 32 within which is disposed a 
valve element 33 urged against a conical valve seat 34 in the housing by a 
spring 35 acting acting between the valve member and an abutment, shown as 
a circlip 36. The valve element has an axial through bore 37 of very 
narrow diameter in order to provide a leakage path through the valve, for 
the purpose to be described. 
A plurality of springs 38 are mounted in the end wall of the housing 1 and 
act between that end wall and the opposed wall of the piston 27 so as to 
apply a permanent pre-load across the clutch 16 via the thrust bearing 28 
and belville stack 24 so that, as wear of the clutch facings takes place, 
tending to produce extension of the belville stack, the piston 27 and 
clutch member 19 are urged to the right by the springs 38, maintaining the 
stack 24 in substantially its original state and adequate clutch engaging 
force thereby continues to be provided. It would be possible to use only a 
single pre-loading spring, which may conveniently be a coil spring 
surrounding the shaft 12 and inserted within an annular groove of the 
piston. 
The input shaft 12 is continuously rotated by a power source, such as the 
engine of a vehicle in which the system is installed. The chamber 26 is 
initially unpressurized and the conical washer assembly 24 clamps the 
clutch plates into firm engagement, enabling drive to be transmitted from 
the input shaft 12 via the clutch member 19 to the crank shaft extension, 
causing reciprocation of the piston 3 and charging of the reservoir 31. 
When the pressure within the reservoir reaches a predetermined value, the 
governor valve 30, which is responsive to the reservoir pressure, operates 
to apply the reservoir pressure via the control valve 29 to the chamber 
26. The area of the piston 27 is chosen, in relation to the applied 
pressure and force of the washer assembly 24, so that the reservoir 
pressure applied to the piston produces sufficient force to overcome the 
washer assembly 24 and thereby pushes the clutch member 19 to the right, 
as viewed in the drawings, against the action of the washer assembly 24 to 
disengage the clutch. This interrupts the drive between the input shaft 12 
and the compressor piston and prevents further charging of the reservoir 
until this is required according to the conditions of use. 
As usage of the braking system takes place, the pressure in the reservoir 
will be progressively reduced and will ultimately reach a predetermined 
pressure at which the governor valve is set to operate to disconnect the 
reservoir from the chamber 26. The pressure in this chamber will now 
gradually decay through the passage 37 in the one-way valve 29 so that the 
clutch disengagement force exerted by the piston 27 is gradually reduced, 
enabling the conical washer assembly 24 to re-exert a clutch engagement 
force in a progressive and controlled manner, thereby avoiding excess 
shock loadings being applied to the compressor components, as would occur 
if the clutch were permitted to be abruptly engaged. During normal 
operation of a vehicle in which the system is installed, the cycle will be 
repeated to maintain the pressure in the reservoir 31 within a range 
between desired maximum and minimum pressures. 
In this embodiment, it will be seen that, because the clutch member 19 and 
drive input shaft 12 are arranged to rotate as a unit, the only components 
of the drive train (excluding the crank shaft) which are required to be 
accelerated upon clutch engagement are the driven clutch plates 20, which 
means that the inertia of the driven components is minimized, enabling a 
lower capacity clutch to be used than that required, for example, in the 
arrangement described in our British patent application No. 8306874. 
The arrangement shown in FIG. 2 of the drawings is generally similar, both 
in structure and operation to the arrangement described above and only the 
significant differences will be described in detail. 
The principal differences lie in the arrangement of the clutch components 
relative to the drive input shaft 12 and crankshaft extension 9. The 
clutch member 23 no longer forms part of the input shaft 12, but is 
mounted on the crankshaft extension 9 inboard of the clutch 16, being 
keyed at 9A to the extension and rotationally supported relative to the 
input shaft 12 by way of a tapered roller bearing 40. The clutch member 19 
is mounted on an axially extending boss 23A of the clutch member 23 and 
surrounds the member 23 with its open end facing the drive shaft 12. The 
clutch member 23 is in splined engagement, as previously, with the 
interior of the member 19. In this embodiment, the clutch 16 is disposed 
at the side of the member 23 remote from the crankshaft and the piston 27 
at the opposite side thereof, the washer assembly 24 being preloaded, as 
before, by one or more springs 38. 
In this embodiment, the drive input shaft 12 is mounted relative to the 
sub-housing 14 by way of a further tapered roller bearing 41 retained in 
position by a circlip 43, and the driving clutch plates 17 are mounted 
directly on the shaft 12 by way of splines 18, the driven clutch plates 20 
engaging the clutch member 19 by way of the splines 21 with which the 
clutch member 23 is engaged. A shim 44 may be provided between the bearing 
41 and circlip 43 to take up end float in the bearing/shaft assembly. 
The direction of tapered of the taper roller bearings 40, 41 is in opposite 
directions relative to the longitudinal axis of the shaft 12. The bearing 
41 reacts axial load occurring in the clutch during disengagement thereof 
and the bearing 40 reacts axial load generated when drive is applied to 
the drive input shaft 12 by means of a helical gear drive, the helix angle 
of which would be typically 20.degree.. 
It will be understood that various constructional details of the 
embodiments herein described may be modified, without departing from the 
scope of the present invention. For example, as described in our 
co-pending British patent application No. 8215732, it would be possible to 
drive the piston via an eccentric in place of the crank shaft 5, thereby 
enabling a constantly rotating shaft to be provided for driving auxilliary 
equipment, as described in our aforesaid application. Various mechanical 
details, such as the type of clutch may be varied as desired and, although 
the clutch is illustrated herein as being contained within a separate 
housing 14, the latter may be formed integrally with the main housing 1 of 
the compressor. Although FIG. 2 shows the control valve 29 as a separate 
item, it may be incorporated in the housing 1, as in the arrangement of 
FIG. 1. 
In the FIG. 2 embodiment, the control valve 29 is shown as a separate item 
arranged externally of the housing 14. The valve 29 would conveniently be 
identical with that described in relation to FIG. 1 and incorporate a 
leakage path in the form of passage 37 for the piston pressure. It would, 
however, be possible for details of the valve to be varied from the form 
illustrated. It would also be possible in any embodiment of the invention, 
to incorporate the leakage path as an item separate from the valve 29, 
either within the compressor housing or in a separate component.