Concern over the Tomkins modification to the BC box

8 Mar

In a recent TTT 2 article advocating the use of the Tomkins modification on a rebuilt Bishops Cam steering box, an addendum by the editor warned of possible safety consequences.

It seems that there is a need to clarify why the Tomkins modification places an increased stress on the sector shaft arm that would not have been considered in the original design.

A particular feature of the Bishop’s Cam box’s design is the tapered peg running in a compatible tapered groove machined in the worm gear. This feature enables the mesh between the peg and the groove to be adjusted by placing shims between the box body and the top plate. The total shim thickness is selected to give a minimum running clearance between the gears in the straight ahead position as defined by a slight “pinch effect” when turning the steering wheel.

The tapered peg’s profile has the undesirable effect of creating an upward thrust as the peg tries to climb out of engagement with the worm’s groove during either rotation of the worm gear whilst steering, or deflection of the tyre hitting a pot hole, for example. This upward force on the peg is transferred to the top of the sector arm directly above the peg and countered by the top plate.

The contact between the sector arm and the top plate is a significant source of friction, which was intended to be overcome by the use of a needle roller thrust bearing in the Tomkins modification.

The needle roller bearing can only be positioned on the axis of the sector shaft, and due to the limitations of the surrounding housing, the bearing cannot extend to cover the sector arm directly above the arc of the peg. The result is a bending moment on the sector arm (Fig.1).

In the specific case shown in the photo below, the bending moment introduced by the Tomkins modification has caused the sector shaft to fail.

A Tomkins modified 70 year old Bishops Cam box may be unsafe if the original sector shaft is retained. If you choose to use the Tomkins modification, then you should at least replace the sector shaft with a modern high quality, steel alloy version, which may be capable of resisting the bending moment.

To repeat, the additional force introduced by the Tomkins modification has caused the sector arm to fracture away from the shaft as shown in the above photo. A Tomkins modified, 70 year old Bishops Cam box is a potential liability. If you insist on continuing to use the Tomkins modification, then one should at least replace the sector shaft with a modern, high quality steel alloy version.

Eric Worpe


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