Bristol Jupiter VIII F., VIII F.P., XI F., XI F.P.
Part 3: Lubrication and Induction Systems
Compiled by Kimble D. McCutcheon
Published 1 Apr 2025; Revised 3 Apr 2025

 

 

Lubrication

The lubrication system delivered oil under pressure to the master rod big end and wrist pin floating bushes, the cam gear, the reduction gear, the magneto drives, the hand-turning gear and fuel pump drives. The pistons, connecting rod small ends, crankshaft main roller bearings, tappets, cam faces, reduction gear parts not lubricated by pressure, and the gas starter distributor were lubricated by indirect or splash oil draining from the pressure-fed components. The valves received a certain amount of lubrication by oil that passed the pistons. The rocker arm bearings were greased by hand at regular intervals. All oil drained from the engine interior was collected in a small sump at the crankcase bottom and withdrawn through the sump filter by a suction or scavenger pump, which drew it through the carburettor oil jacket and passed it to the aircraft tank. In order to ensure a dry sump the scavenger pump capacity was approximately one-third greater than the pressure pump's. Both pumps were the meshed gear types and were built into a single unit.

The oil pump unit was housed in a rear cover chamber; the hollow crankshaft rear end opened into the chamber, which provided a simple means of introducing oil into the shaft. All the scavenger oil passed through the left-hand filter in the rear cover before entering the scavenger pump, and oil from the aircraft tank passed through the right-hand filter before entering the pressure pump. The filters were easily accessible for cleaning by unscrewing the hollow plugs to which they were attached and which closed the rear ends of their respective chambers. The pump unit, which comprised the body for the pressure pump, the scavenger pump and the pressure relief valve, was built up in sections that were clamped together between two end plates by four long studs. The rear end plate was flanged for attachment of the unit to six studs around the pump opening in the rear cover; the joint was made oil-tight by a synthetic leather ring. The rear end plate provided a base for the assembly and also housed the relief valve and a union for a pressure gauge connection. The cylindrical pump unit's exterior surface was machine finished for a push fit into the rear cover chamber. The various inlet and outlet passages in the rear cover converged towards the pump and these passages broke through the chamber walls to register with ports in the pump body walls and thus conduct oil to and from the pump. A small amount of leakage occurred but was confined to the chamber interior; no external leakage occurred. The pump body did not extend to its chamber bottom in the rear cover, but left a pressure chamber for oil at the cover front end. The main supply to the crankshaft was delivered from the pressure pump through both pumps' idler gears' hollow spindles into the pressure chamber. Since the crankshaft end was open except for a coupling plate by which the pumps were driven at engine speed, the oil was able to pass directly into the crankshaft bore.

The pump unit construction was simple in design, easy to access and easy to assemble; one of the four studs was larger than the remainder in order to index the components and ensure correct assembly. The pressure pump was behind the scavenger pump. Each pump casing consisted of a solid duralumin disc in which were machined the housings for the drive and the idler gears. The two pumps' capacity difference was obtained by gear length differences and casing thickness. The pressure pump inlet port was cut in the casing on the pump's starboard side while the delivery port was cut in the rear end cover wall on the pump's pressure. The scavenger pump inlet port was also on the starboard side and the delivery port on the port side. The pressure and the scavenger pumps were separated by a steel plate, which, like the housing, was threaded over the studs; similar but thinner plates were interposed between the gears and the unit's end plates so that both pumps' end walls were steel lined.

The iron-nickel-aluminium-bronze alloy pump gears were mounted on two hollow spindles that extended the unit's length. All gear teeth were relieved, i.e., one face of the tooth was reduced in thickness, so as to release oil that would otherwise be trapped between the tooth top on one gear and the space between two teeth on the other. To ensure the gears were correctly engaged in relation to one another, the idler gears were marked with a cross at the end adjacent to the central plate dividing the pressure pump from the scavenger pump. An index plate to this effect was be found on the pump. No marking was necessary for the driving gears, as they could not be incorrectly assembled on their spindle.

The uppermost or driving spindle had two grooves from end to end that engaged internal dogs in both pumps' formed driving gear bores. The spindle protruded through the front end plate beyond which it was formed with a flange, which was machined with two slots that engaged the two dogs in the magneto driving bevel sleeve end. The idler spindle diameter was larger than the driving spindle; in addition to acting as a spindle it served as a delivery passage for oil from the pressure pump.

Items assembled on the driving spindle were drawn up by a small nut at the shaft  rear end, making them integral with the shaft. At the same time the assembly enabled the correct gear and spindle end float to be obtained in a very simple manner. The driving spindle forward end was stepped up to provide a journal that ran in an unbushed boss in the front end plate, supporting the journal load at that end. The scavenger gear abutted the shoulder formed by the journal but the hole in the steel end plate at this end of the pump was large enough to admit the journal. A step formed on the scavenger gear rear end aligned with the intermediate steel partition dividing the two pumps and served as a spacer between the scavenger and pressure gears. The hole in the partition cleared the spacer, and its depth, being slightly greater than the partition thickness, provided the necessary end shaft  clearance when the two gears were drawn up against each other. At the pressure pump gear rear end the bore was enlarged to admit the flange of a steel cap against which the nut was attached and which also served as a journal for the spindle at this end and ran in a bush in the rear end plate. The nut was locked by a split pin. The front and rear spindle journal loads were borne by the front journal and the end cap respectively, whilst the axial location was determined by the spacer thickness on the scavenger gear.

The idler gears were freely mounted on their spindles and were located by the central partition and pump body end plates. The spindle was a driving fit in the unit's front and rear end plates. The spindle rear end had a shoulder that abutted an extension in the rear end plate formed in the chamber bottom that accommodates the relief valve and the relief valve seats in the hollow spindle end. Holes drilled radially in the spindle immediately in front of this coincided with the recess formed around them in the end plate and passed oil from the pressure pump into the spindle's hollow bore from where it flowed out at the front into the oil space between the pump unit and the chamber bottom and then to the crankshaft open end. The rear end plate had a passages that conducted oil to and from the relief valve and to the pressure union. Details of this will follow in the circulation description.

A small check valve at the hollow idler spindle forward end prevented oil flowing from the tank, through the pump, and into the engine in installations where the oil tank was situated above the pump. The spring loaded check valve opened at 5 psi. It was carried by a small bridge piece fitted across the pump bottom and secured at each end on two of the studs. The relief valve was machined with a 45° face that registered with a corresponding seating in the hollow idler gear spindle end. Three equally-disposed vanes projecting from the valve stem located it in the hollow spindle bore. A spigot on the valve head carried a coil spring accommodated in an aluminium plug that screwed into the pump rear cover plate back. A tapped hole in the plug top carried a flat headed screw the stem of which projected outside the plug and was formed with a squared end for operation with a spanner. The relief valve spring was captured between the valve head and the forward screw head. A locking nut on the screw fixed it in position once it was adjusted, and a hole drilled in the screw end accommodates a locking wire anchored to a drilled lug on the adjacent pump body.

A small groove on the rear end plate inner face connected the hole for the driving spindle with that of the idler spindle; two small holes in the adjoining shim registered with each end of it. This was provided prevent oil under high pressure in the pressure pump delivery side from forced its way past the spindle hole in the shim and builing up a pressure beneath the shim. As the shim was thin and unsupported at the center, buckling occurred, which caused the shim to bind slightly on the gears. The provision of the groove and the holes allowed the pressure beneath the shim to be released and to pass away by the inlet side of the pump.

Pressure Pump Circulation. Oil entered the rear cover by the left hand union underneath the pressure filter chamber plug, passed through the filter, left the filter chamber by the passage on the chamber starboard side, and via an extension of this passage machined in the pump chamber wall, reached the pressure pump inlet port. A staggered communication cavity was provided here as the passage did not break into the chamber in line with the pump inlet port. After passing through the pump the oil left by a port in the pump housing rear wall, through a passage in the pump rear end plate and into the annular space surrounding the idler gear spindle hollow end; the pressure gauge union was in the end plate passage. The oil passed through the holes in the spindle and along its bore to the space between the pump end and the chamber bottom, where it entered the crankshaft open end. When oil began flowing and pressure built up in the system, the relief valve, which seated in the hollow spindle rear end, was raised from its seat and the oil passed into the valve housing. This communicated by a passage with the pressure pump inlet side and again passed the oil to the pressure pump inlet side; this procedure continued until in the oil temperature rose sufficiently to reduce the pressure and allow the relief valve to nearly close under  its spring pressure. The relief valve never completely closed when the pump was operating and maintained a constant pressure throughout the system.

Scavenger Pump Circulation. After passing the sump filter, oil was drawn through the carburettor jacket and then entered the rear cover by the scavenger, or starboard filter union. After being drawn through the filter chamber it left by a passage into the pump chamber starboard side and registered with the scavenger pump inlet port. Oil left the pump by the outlet port on the opposite side where the port registered with a staggered cavity in the pump wall, which brought it into contact with a passage that broke into the chamber wall at the bottom. This passage led to the central union on the rear cover to which was connected the return pipe to the aircraft oil tank.

Engine Circulation. Oil delivered into the space at the oil pump chamber bottom was forced through the crankshaft bore to the various assemblies. A proportion also flowed directly from the pump chamber to the two slots in the metal surrounding the magneto bevel drive sleeve white metal bearing where a duct was drilled to the center of each magneto drive spindle bearing. Another duct drilled at the recess top led to the layshaft plain bearing, which was situated immediately above the oil pump. This passage led into the space between the two bushes that constituted the layshaft front bearing, and after lubricating the bearing, passed out of the space by the flat on the shaft, providing the intermittent feed to the hand turning gear and revolution indicator drive. After passing the flat the oil left by a two-way swiveling union on the casing; one pipe ran to the hand turning gear casing and the other to the fuel pump drive. The hand turning gear oil returned to the rear cover by two passages in the hand turning gear casing bottom, which registered with two corresponding holes in the cover. Oil returned from the fuel pump drive by an external pipe that enters the rear cover by a union in its lower segment.

The main engine supply was forced along the crankshaft rear portion, through the duct drilled up the crankshaft rear web and into the hollow crankpin. Two radially drilled holes in the crankpin opened onto a flat on the crankpin and supplied oil to the master connecting rod big end floating bush inner surface. Numerous radius holes in the bush passed the oil to its outer surface and ensured efficient lubrication. Oil leaving the big end bearing was trapped at each end by the thrust collar and oil retainer, which were fitted on the rear and front ends of the bearing respectively. The majority of the oil was trapped in the annular recess between the oil retainer and the floating bush and then passed through the eight ducts drilled in the big end to the eight wrist pins. Entering the compartment formed between the wrist pin bore shoulder and the cup at the wrist pin front end, by a circular groove drilled with two holes, the oil passed from this chamber through two oblique holes to the two flats formed on the wrist pin surface, providing wrist pin floating bush lubrication. The fitting of the oil retainer and thrust ring did not entirely preclude oil leakage from the master rod ends, and the leakage that occurred, along with oil leaking from the wrist pin bushes contributed to the splash supply by which various components were lubricated.

Oil flow in the crankshaft continued down the passage drilled in the crankshaft front web, across the shaft center line, to a duct drilled out of the web into a radial space between the main roller bearing's chamfered bore inner ring and the web. From there it passed into the three broad shallow oilways formed on the shaft front portion, which, being covered by the components installed on this length of the shaft, were converted into reservoirs. The cam sleeve was lubricated by oil from these reservoirs, which reaches it through a groove and holes in the crankshaft sleeve; these supplied the recess between the two cam sleeve bush halves, where the oil was distributed over the whole surface of the bushes by the spiral grooves cut in them. Part of the oil thrown from here lubricated the intermediate main bearing. The pinion and its floating bush were lubricated by oil from holes drilled radially in the eccentric which were supplied by two recesses in the eccentric bore that were open to the oil reservoirs on the shaft. The bush was drilled with numerous holes that ensured a liberal distribution between the pinion and bush. Oil thrown out from the pinion, together with that from the cam sleeve, lubricated the remainder of the cam gear, the tappets and also the front main crankshaft bearing. A semi-circular sheet aluminium oil baffle on the front cover foremost side prevented excessive oil leakage at the breathers. The gas starter distributor was supplied with oil from an oil chute at the baffle top, which routed oil through holes in the baffle and delivered it by a smaller chute at the other side on to the distributor drive housing top. Part of the oil enters the drive housing where a drain hole maintained the level at the correct height to lubricate the drive, while the remainder passed up the spindle to lubricate the distributor spindle ball bearing.

Scavenge Oil. Surplus oil from the front main crankshaft bearing and cam gear drained into the front cover through three holes in the fixed internal gear plate and then through a vertical crankcase passage that registered with the foremost sump inlet. Drain oil from the crankcase interior flowed out at the bottom through two holes at the front and two at the rear, which communicated with the front and rear sump limbs. Two drain holes at the crankcase rear wall low point passed all the drain oil from the assemblies mounted on the rear cover. In order to prevent the oil thrown from the revolving crank web from obstructing drain oil passage, the holes were shrouded by a small aluminium cowl or louver. A trough fitted underneath the crankshaft immediately behind the rear main crankshaft roller bearing collected oil and ensured libera1 bearing lubrication. Surplus oil from the hand starter gear drive was returned by two holes in the gear casing that registered with two others in the rear cover wall where the oil finally drained. Two drains in each magneto drive housing passed the oil to the rear cover front and then to the crankcase by the drain holes in the rear wall.

The articulated rod and gudgeon pin small ends, along with the pistons, intermediate and rear roller bearings were lubricated by oil thrown off from the master rod big end. The reduction gear oil supply was taken from the duct in the crankshaft foremost web and passed through a baffle fitted in the crankshaft front bore to reduce the oil flow and pressure. This was necessary to prevent excessive gear lubrication while still maintaining the correct main system oil pressure. Having passed the baffle the oil filled the crankshaft and airscrew shaft bores; a certain quantity entered the airscrew shaft bush in the crankshaft front end, and grooves were cut in this bush to ensure distribution over the whole of its surface. The remainder of the oil passes out of the airscrew shaft bore, up the ducts drilled in the bevel pinion stub centers and then out by ducts drilled radially to the spindle axes. Each duct broke into a flat machined down the side of each spindle that distributed the oil over the bearing's length. The duct was drilled towards the stub spindle inner end so that the oil passing out of the duct would flow the length of the flat by centrifugal force and thereby ensure adequate lubrication. The rest of the reduction gear working parts were oiled by splash. A spring-loaded oil retaining gland was fitted immediately in front of the airscrew shaft ball bearing to prevent escape of oil from the casing front. Oil thrown off the gear collected in the gear casing where it was maintained at the requisite level to form an oil bath into which the gears dips and by which the components not included in the pressure system received splash lubrication. The level was controlled by the external overflow pipe that ran from a union situated slightly above the casing's lowest point to a union in the sump front wall. The pipe, which was in two lengths to prevent fracture through vibration, was joined midway by a hose connection. The hose was bound with brass gauze and the clips which secured this and the hose were connected by flexible copper wires to the adjacent pipe halves, thus providing electrical bonding.

The sump was fitted with a removable filter housed in an open-ended chamber, formed on the sump floor. The strainer consisted of a cylindrical cage lined with brass wire gauze. At the cage forward end a threaded shank screwed into a union at the sump's lowest point, a hexagon and flange being formed at its foremost end to receive a spanner for removal and insertion. The open-ended chamber into which the filter barrel fitted was cut short of the front wall, thus exposing the barrel, which was here drilled with four holes that, together with the open rear end, provide the inlet to the filter. The oil was withdrawn from the filter chamber by the suction pipe from the scavenger pump, which enters the sump back by a union nut and which ran into the chamber. The filter's hexagon head was locked in position by a spring clip. A round bar magnet, housed within the filter, collect ferrous particles present in the oil. The bar was installed from the filter rear end and screwed into a tapped boss in the wall of the castellated plug at the front end. The magnet was locked by a lock nut which was screwed on to its forward end which projected through the plug. A helical steel spring fitted on the magnet and extending over most of its length acted as a deflector that directed the oil over the magnet's surface.

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