Making light weight composite camshaft with Trib-Joins
Application Note 15.
The illustration shows a built-up camshaft assembled with press fit joins strengthen by a cold welding agent applied with a Trib-Tool. Examples of the individual pre-formed parts used in its construction are also shown alongside. The shaft is easily assembled by hand with the aid of a simple alignment jig and hydraulic clamp. The fist step is to establishthe timing reference face by pressing in and welding the drive spigot. The cams and journals are then forced along the dry shaft in the correct sequence from the other end. Each cam is individually staked into place with treated jams, the bearing journals are pressed on in turn and finally the auxiliary drive end insert is pressed home. This method is ideal for development and competition shafts where speed and flexibility are vital time savers. Equally there is no reason why this method cannot be automated and used in volume production
The tube is a welded and drawn over mandrel (DOM) 25 O/D and 20 I/D standard steel stock. It may need a lick with a grinder to establish the outer diameter and straightness. The bearings are hard press fits and are pre-positioned dry. They are cold welded in place by treating the final position with the Trib-Tool, which gently abrades the surface clean and at the same time releases trace amounts of cold welding agent, which becomes absorbed into the oxide layer on the shaft.
The cam lobes are made from pre-ground bearing grade steel tube stock. They are parted off as soft rings and then roll formed to the desired shape. By rolling, residual bending stress can be virtually eliminated at the critical apex. The lobes are then hardened using processes similar to that used in bearing manufacture.
The problem of maintaining shaft straightness during assembly is solved by staking the lobes individually in position with soft steel welding jams, thus avoiding the need to apply large axial stress to the unsupported shaft. The jams are small interlocking segments fine-blanked to match the inside profile of the lobe. The jams are slightly oversized height wise and deform the tube very slightly to take up manufacturing tolerances when inserted, but the deformation is not sufficient to affect straightness. The jams are treated with trace amounts of cold welding agent and cold weld to the tube and lobe as they are forced into position to provide a good secure anchorage. In the illustration three 3 mm wide jams are used to secure the 12 mm wide cams. The inside of the cam lobes may also be treated to enhance grip between the lobe and the shaft. The shaft yields at 300 N.m. in torsion and the cam joins are capable of withstanding a similar force. In service a typical torsional load is about 20N.m. in a two litre automobile engine. The tube thickness was determined by the torsional stiffness needed for the overall assembly.
Finally the slightly oversize auxiliary drive end insert is pressed into the end of the shaft after treating. This is sized to provide a torque strength of about 200 N.m. so if the driven auxiliary seizes the end join slips but the camshaft is not destroyed as the engine comes to a stop. Trib-joins have the unique ability to yield in cases like this without loosing their strength - in fact they actually increase their strength slightly as they absorb energy.
The assembled shaft only requires a minimal finish grind. It saves typically 1 kilo per shaft weight and the cam positions and profiles may be varied with considerably more freedom than is the case with conventional cast iron shafts. All the joins are steel to steel cold asperity cold welds and are unaffected over life by oil and temperature (they tend to increase in strength with age due to diffusion). Laboratory and engine tests on similar camshafts using a hard steel band showed no sign of abnormal surface wear or sub surface Hertzian fatigue.
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