ERIKS UK´s Motorsport division in Oxford were called upon by John Piper, JCB Dieselmax Project Chief Designer to supply a large diameter bearing solution with a very narrow section for use on the innovative wheel hubs of the record breaking JCB Dieselmax car.
The bearings supplied had to meet stringent packaging and performance requirements in order to facilitate the innovative JCB DIESELMAX dual-circuit, triple braking system comprising friction brakes on all four wheels, driver-activated engine braking, and parachutes. The bespoke wheel braking system employs carbon ceramic brake rotors clamped, not by conventional six-pot racing callipers, but instead by brake pistons mounted within the wheel upright.
Geoff Cox, Regional Manager ERIKS UK comments on the tough design requirements the bearings had to meet, "The wheel axles are hollow on this car, similar to those on Formula 1 cars - the air gap is used for cooling purposes, allowing air to reach the centre of the brake assembly. Tubular axles are also lighter and have a higher torsional rigidity when compared to thinner solid shafts. The width of the shafts combined with low-weight requirement and a small operating envelope meant that machine tool bearings were the only option."
The brake pistons are activated by a torque tube that pushes them hydraulically into contact with a stator that in-turn clamps the wheel-driven rotor. The system provides enhanced swept area and effectiveness with the aim of not just providing conventional braking force but a friction brake system capable of stopping the car in an emergency, such as complete failure of the twin parachute back-up system.
The axle shaft diameter is 120mm and with only a 165mm outside diameter and a 22mm width available for each bearing, the tight packaging meant they were not a straightforward item to source. ERIKS UK ultimately supplied very thin section super precision angular contact bearings manufactured by SKF, the bearings employ ball bearings rather than taper rollers as these are smaller and more efficient, providing far less rolling resistance.
The bearings supplied are the lightest series available in these dimensions, but are over specified in terms of speed rating, the high load offsets this though; the wheels are only spinning up to 5500 rpm on a record attempt whereas the bearings will comfortably reach over twice this, but carrying an overall weight of 2700kg including fuel, oil, ice and water coolant and driver.
John Piper points out: "The car is four times as heavy and almost twice as fast as a Formula One car, so there is a lot of mass to stop and a great deal of heat to dissipate. This system enables us to get as big a brake as possible within the 15 in wheels, which will be turning at twice the speed of rotation of a Formula One car´s."
The bearings are mounted in pairs and the amount of pre-load is judged by the team based on the load, the speed and the lubrication method employed. The bearings were supplied open, but packaged to clean room conditions allowing the team to apply it´s own design of low-friction sealing element.
John Piper is Project Chief Designer on the Dieselmax project.
Geoff Cox is Regional Manager of ERIKS UK Industrial Services, and a bearing expert with a wide range of industrial and Motorsport experience.