GB2179417A - Regulating pumps - Google Patents

Abstract

A piston and cylinder air compressor has a connecting member 11 between a crank pin 10 of a crank-shaft 8 and a piston 6 which connecting member comprises a piston 26 and a piston rod 28 connected to the crankpin and a cylinder connected to the compressor piston 6. Lubrication oil is admitted to the cylinder bore 27 to maintain a rigid connection with piston 26 immobile relative to cylinder 27 when the compressor is to operate. When oil is drained from the cylinder via passages 29, 17, 25a and 25b, the piston can "free-wheel" inside the cylinder so no compression occurs. Control may be achieved by supplying a signal pressure at port 19 to move valve 21 to the right for draining the cylinder. In the absence of such a signal spring 20a holds the valve in the illustrated position so that oil pressure may be supplied via port 16. <IMAGE>

Description

SPECIFICATION Reciprocating machine This invention relates to a reciprocating machine and relates especially but not.exclusively to a piston and cylinder air compressor.

Piston and cylinder air compressors are commonly used in heavy road vehicles or rail vehicles with compressed air braking systems.

Especially in heavy road vehicles such compressors are usually driven via suitable gearing by the vehicle engine and serve to charge one or more storage reservoirs for supplying independent braking circuits for the vehicle. In order to ensure charging of operative reservoirs in the event of failure of another reservoir or associated circuit, the reservoirs are normally charged through suitable circuit protection valves whereby the operative reservoir or reservoirs is assured or a predetermined level of charged pressure.

It is normal to provide such reservoir charging systems with a pressure responsive governor operable to control an unloader valve or a clutch whereby the compressor stops charging the reservoirs when a predetermined value of pressure is attained. It is recognised moreover that although an unloader valve is usually a simpler and cheaper device than a mechanical wet-plate or dry-plate clutch, a clutch interposed between drive means and the compressor, has the advantage that the crankshaft and piston assembly can be stationary during periods when no charging of the reservoirs is required. One benefit of this is that oil carryover into the compressed air system may be appreciably less with a clutch driven compressor than with a compressor which uses an unloader valve.

Having regard for the extra space and complexity involved in incorporating a wet or dry plate clutch into a compressor drive, the present invention seeks to provide a reciprocating machine with an alternative means for disconnecting a rotary drive means from a reciprocating member.

According to the present invention there is provided a reciprocating machine including a first rotary member eccentrically pivotally connected via a connecting member to a reciprocating member whereby rotary motion to the first member is translated into recirpocating motion of the second member or vice versa said connecting member including means whereby it is selectively rendered rigid or freely extensible to respectively connect or disconnect the drive between the first member and the reciprocating member.

In order that the invention may be more clearly understood and readily carried into effect, the same will be further described by way of example as applied to a piston and cylinder air commpressor with reference to the sectional diagrammatic view of an air compressor as shown in the accompanying drawing.

Referring to the drawing, the air compressor comprises a crankcase and cylinder housing denoted by reference (1) provided with a valve plate (2) and a cylinder head (3) with input and output ports (4 and 5) respectively.

A conventional piston (6) is provided to reciprocate within the bore (7) of the compressor cylinder. A crankshaft (8) has main bearings such as the bearings (9) as shown and a crankpin (10), pivotally connected via a composite connecting member (11) to a more or less conventional small end bush (12), a gudgeon pin (13) being located in the piston (6) by circlips (14).

The bearing (9) is carried in a plate (15) which incorporates a pressure responsive valve assembly via which oil under pressure presented from the vehicle lubricating oil circuit to a port (16) is communicated via a valve arrangement and a bore (17) in the crankshaft (8), to an annular recess (18) provided in the large end bearing. The valve assembly carried by plate (15) has a further port (19) which communicates with one side of a valve-controlling piston (20) biassed leftwards by a captive spring (20a). The piston (20) has a stem (21) which passes through a seal through a seal (22) and at the inner end carries a valve closure member (23) which under the influence of spring (21) rests against a valve seat (24) to close a passage (25b) between port (16) and the crankcase.A further passage (25a) exists between the port (16) and the adjacent end of the crankshaft (8) whereby with the valve in the position showing oil under pressure at port (16) is communicated to the composite connecting member (11) which is about to be described. The connecting member (11) comprises a piston part (26) connected to the big-end and sealingly slideable in the bore (27) of a cylinder part.

The piston part (26) has a stem (28) with an internal passage (29) via which the passage (29) and the annular recess (18) communicating with the interior of the cylinder bore (27), the stem (28) being sealingly slideable in a gland (30) in a screw-in closure part (31) at the lower end of the cylinder.

In operation of the air compressor, the crankshaft (8) is driven through a gear train (not shown) by the engine of a vehicle. Oil under pressure is applied at the port (16) by the engine lubrication system and with the piston (20) and valve closure member (23) in the position shown, this oil pressure is applied via the passage (25a) the end of the crankshaft and passages (17) and (29), to the interior of the cylinder bore (27). This piston part (26) and cylinder are therefore retained by this pressure in the condition shown and the connecting rod operates as a solid connecting rod. When a govenor in the compressed air system senses that the or each reservoir is fully charged, a signal pressure is thereby applied to the port (19). This acts on piston (20) to overcome the closing force of spring (21) whereby the valve member (23) lifts off the seat (24).The oil pressure in the cylinder (27) is thereby relieved via passages (29, 17, 25a and 25b.) This permits the stem (28) to move downwards in the gland (30) during an ensuring downward movement of the crank pin (10) and as soon as any trapped air pressure in the compression chamber above the piston (6) has leaked away the piston (6) becomes more or Iress stationary. The compressor thus ceases to operate since the piston (26) freely reciprocates in the cylinder bore (27). When the reservoir pressure descends to a lower value the signal at the port (19) is removed by the govenor and the valve member (23) recloses over seat (24) whereby the oil pressure at port (16) is reapplied to passage (25a) so that piston (26) is again constrained to take up the position shown in the cylinder bore (27). The compressor thus resumes normal operation.

It has to be recognised that with a piston and cylinder compressor such as described in the foregoing the oil ways and the viscosity of the oil fed into the cylinder (27) may be such that this cylinder may not be scavenged within a single stroke of the piston 20 being actuated. The assembly will, therefore, require to be mechanically capable of accepting several partial piston strokes when going on or offload.

Since the connecting rod assembly is pressurised by oil which also provides lubrication of the big-end bearing, it is also necessary to ensure that the pumping effect of air which takes place in the connecting rod when the compressor is off-load or idle, does not result in the bearing running completely devoid of lubrication. In order to avoid this the controlling valve arrangement may be adapted if required to provide oil spray to the crankcase during off-load periods.

I order to avoid unnecessary operation of the oil pressure relief valve which is usually included in the lubrication oil pump circuit it is desirable to include a check valve in the supply line to port 16 of the control valve Such a check valve will prevent excess back-pressure, as produced by the piston 26 in passage 17 reaching the relief valve in the oil pump circuit.

Claims (5)

1. A reciprocating machine including a first rotary member eccentrically pivotally connected via a connecting member to a reciprocating member whereby rotary motion of the first member is converted into receiprocating motion of the second member or vice-versa said connecting member including means whereby it may be rigid or freely extensible such as to inter-connect or disconnect the first member and the reciprocating member-for such motion.

2. A reciprocating machine as claimed in claim 1, wherein the connecting member comprises a cylinder part connected to one end thereof and a pressure responsive part sealingly slideable in the cylinder part and being connected to the other end thereof with means for selectively admitting to or releasing fluid under pressure from the cylinder part on one side of the pressure responsive part.

3. A reciprocating machine as claimed in claim 2 said fluid under pressure acting in a sense to urge said ends towards one another.

4. A reciprocating machine as claimed in claim 3 being a piston and cylinder type gas compressor and said fluid under pressure being supplied via passageways from a lubrication system thereof.

5. A gas compressor as claimed in claim 4, the fluid under pressure being supplied via a large end bearing of the connecting member.