From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Detailed analysis of the Bentley Mark V chassis, focusing on the gearbox, hypoid rear axle, and braking system.
| Identifier | ExFiles\Box 160\5\ scan0332 | |
| Date | 1st May 1941 | |
| 148 AUTOMOBILE ENGINEER THE BENTLEY MARK V CHASSIS (Continued) before the chassis went into production. Formed integral with the bevel pinion, the driving shaft is unusually well supported by a compound bearing as seen in Fig. 11. In the centre is a ring of rollers, and on each side a ring of balls having special races capable of adequately dealing with a considerable amount of thrust in both directions. The whole bearing, which is very rigid in a radial direction, is housed in a special sleeve mounting, the flange of which is sandwiched between the flange on the gear box casing and the end oil retaining cover. The two members are retained by a ring of fine studs. In addition to this compound bearing, the pinion shaft is extended and supported by a straddle bearing containing two rows of rollers. Shouldered studs are employed to attach the crown wheel to a flange on the differential cage which is of the four bevel pinion type. Particular interest attaches to the method of mounting the differential cage. Two Timken taper roller bearings are employed, one of these adjacent to the flange side carrying the crown wheel, while the other half of the differential cage is considerably extended and formed with a small flange to take the bearing. The outer race of this bearing is retained in position by a large-diameter washer which is held against the race by a short but robust coil spring. The purpose of this spring is to maintain the preload on the bearing which would otherwise disappear with the expansion of the aluminium axle casing. The axle illustrated, Fig. 11, is of the semi-floating type, the central casing of which is an aluminium casting. This axle was originally intended for the “Corniche” model, while one of the fully floating type was to have been used on the Bentley V.{VIENNA} It may be seen that the axle shaft is carried right through without support to the hub, where it is mounted in a very substantial single-row Ransome and Marles ball bearing. The end of this shaft terminates in a large flange to which both brake drum and detachable wheel are separately secured. By sound design combined with the use of modern extreme pressure lubricants the rear axle has unusual robustness and rigidity. Particular interest attaches to the construction of the brake drum, which is built up of two members, a thin steel dished flange cast into an iron drum formed with five unusually large cooling fins. It is 14in. in diameter. The wedge and roller expander actuation for the shoes, their adjustment and part of the operation linkage are of Girling design. Although equal loads be applied to both brake shoes, with some designs it happens that, owing to unequal wrapping effect, one shoe will exert more pressure, and wear more, than the other. In the Bentley adaptation of the Girling brake, linkage is provided to transfer part of the load from the wrapping shoe back to the trailing shoe. In the end view of the brake included in the illustration of the rear axle (Fig. 11) three links may be seen adjacent to the expander. The first has a turntable adjustment and is attached at the top to the upper shoe. At its lower end it is pinned to one arm of a small balance bar pivoted on a longer rod which has a fulcrum point at its upper end, as shown in the section on line AA.{D. Abbot-Anderson} The other end of the balance bar is pinned to the third link, which in turn is attached to the lower brake shoe. As shown in the sectional view on line BB, the system is preloaded by two coil springs. It is obvious that any movement or force applied to one shoe is automatically equalised between the two. As a result there is equal wear on the two shoes and the effectiveness of the brake is proportional to the coefficient of friction instead of being proportional to a higher power than one. Brake operation is assisted by a mechanical servo (see Fig. 13) which is equally effective in both forward and backward directions. In design the friction servo device is considerably simpler than the mechanism previously used on either Bentley or Rolls-Royce cars. Operated from the gear box by a two-start worm behind the second speed gear, a fabric-lined disc is driven at 1/36 engine speed. On one side of this member is a stationary disc which may be brought into contact with it through the action of cams formed inside an adjacent sleeve. Rollers are interposed to reduce friction. Attached to the sleeve are two arms, the lower connected through pull rods to the brake pedal. Pressure on the pedal causes a slight revolution of the sleeve, bringing the normally stationary disc into contact with the revolving disc. After turning through a small arc, a peg on the sleeve comes in contact with a suitably shaped recess in the lower arm on the sleeve. This in turn transmits a pull to the attached link which is connected to the brake-operating mechanism, supplementing the pressure exerted by the foot. When in reverse, the disc moves in the opposite direction, carrying round with it a third rod having a slotted end. In this slot is a pin tinging it with the brake operating link. When the pin travels to the end of the slot, the servo action operates the four brakes as before. On the opposite side of the revolving disc is a corrugated copper spring which separates the driven and driving discs when the pedal pressure is released. At the same time it acts as a cover keeping out mud and dust. Progressive in action, the servo mechanism is sensitive and instantaneous, the driver’s effort is assisted without any loss of control. For maximum retardation, pedal pressure does not exceed 10 lb. An ingenious arrangement has been devised and patented to ensure that the operation of the rear brake is unaffected by rear-axle movement. Attached to the right-hand rear axle of the cruciform frame member is a depending arm on the lower end of which the front end of the radius rod (see Fig. 12) is mounted through a universal joint. At its rear end this rod is attached to the rear axle tube through a flexible mounting shown in a scrap view in the upper left corner of the rear axle drawing, Fig. 11. This takes the form of a vertical rod, the lower end of which passes through an eye in the radius rod and the upper end is similarly attached to a small cantilever bracket bolted to the axle tube. At both ends two rubber buffers are used so that the mounting is flexible. The radius arm is extended beyond this location point to carry the Girling-type vertical pivot unit, the lower arm of which is connected to the brake actuation arm from pedal and servo and the two upper May, 1941 AUTOMOBILE ENGINEER 149 Fig. 8. Gear box and member for controlling lateral movement of power unit. Fig. 9. General arrangement of four-speed gear box with a direct top drive. Anti-rattle device for 2nd speed synchromesh Fig. 10. Rear axle and exhaust second expansion chamber. Fig. 11. General arrangement of hypoid rear axle and brake mechanism. Section AA{D. Abbot-Anderson} Section BB | ||