# Axial Thrust

I'm in the process of calculating my combined stress on an axle I am designing, I have calculated the bending and torsional stress, however I need to calculate the direct stress in the axle. In order to do this I need to know the axial thrust in my axle, I understand what axial force is but I am unsure what kind of factors need to be taken into consideration when determining what it will be?

I understand that this isn't something which is answered by an equation and is something determined by my environment, design and function.

The axle is designed for one wheel of a wheelchair, supported by 2 bearings and is manually driven at a max speed of 10mph. Not sure if this extra info helps or not.

I think it may have something to do with when going around corners in the wheelchair will cause the most axial thrust but not sure on this either.

Any help would be great and let me know if you need anymore information.

Saints-94

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It sounds like that, in ordinary operation, there will be no axial load on the axle. Axial loads will happen at times, if for example, the wheel chair wheel side-swipes a curb, but this is only occasionally. The fatigue calculations need to be done for the common operating conditions, which seems to be with no direct axial stress, only bending and torsion. (Actually, is there any torsion? Is there torque in the shaft? I don't know because it depends upon how you intend to attach the wheels.)

It sounds like that, in ordinary operation, there will be no axial load on the axle. Axial loads will happen at times, if for example, the wheel chair wheel side-swipes a curb, but this is only occasionally. The fatigue calculations need to be done for the common operating conditions, which seems to be with no direct axial stress, only bending and torsion. (Actually, is there any torsion? Is there torque in the shaft? I don't know because it depends upon how you intend to attach the wheels.)
If I am designing my axle for a worse case scenario, I need to take this possible axial load into account? My initial thought that was when the user is going around a corner, a lot of the users weights and the weight of the wheelchair frame will be transfers down the axle and this would be my worst case axial load?

Baluncore
2019 Award
What if the wheelchair travels part-way up a ramp, then turns sideways to take a rest? That will result in both axial and bending forces. The greatest forces will be carried on the lower wheel.

There will be situations where the entire load will be supported on one wheel, at the same time as an impulse. For example when travelling parallel with a curb and dropping a wheel over the edge.

The greatest axial tension will probably be when the downhill wheel slips on ice and the upper wheel remains in contact with the pavement.

When brakes are applied to the rim, there will be a bending force to the axle.

Saints-94
anorlunda
Staff Emeritus
The worst case scenario could be when the wheelchair falls from a platform and lands on the hub. You probably want to choose a less drastic case.

My point is that the worst case is not a question of physics, but rather a matter of choice.

Saints-94