electric power steering worm application

[bmfrusta]

I am currently working on a project which involves a DC electric motor providing a vehical steering assist torque. The motor shaft and steering shaft will be perpendicular to each other. My question is... Can I design the correct worm gear solution to work for this application? I think I need a relatively large lead angle in order to keep the steering shaft turning when there is no input from the motor. Another idea I had is a clutch on the steering shaft to let the worm gear freely rotate.

Any ideas will be appreciated.

 

[EnglishMuffin]

It sounds as if you want it to be back-driveable. If that's the case - you can achieve that with a worm as long as it has a small enough ratio, but worms are somewhat inefficient. You could also use a crossed axis helical gear pair or various types of bevel gear. Power steering of various kinds has been around for ever, so is there any special reason why you are inventing a new type ?

 

[GearmanPE]

Worm drives can backdrive, depending on the lead angle of the worm. As this angle goes up, the backdriving efficency does as well. High lead angles are acheived with coarse pitch worms with small root diameters and multible threads.

Thread finish, lubrication and materials have significant influence.

 

[EnglishMuffin]

Yes - I suppose I should have said high lead angle rather than low ratio. Dudley's "practical gear design" says self locking can occur at 10 degree lead angle, and more than 6 degrees requires more than one thread, so it would probably mean a two or three start worm and at least 15 degrees of lead angle to be safe. So it would likely be a low ratio in practice.