Rack Rotational Movement

[DEAD_MAN]

Hey guys, ive been through a lot of net searches but couldnt find any thing on How to stop the rotational movement of the rack. Our last years team had made a slot on the rack then with the help of a grub screw stopped the movement, the only drawback we had was the amount of Frictional force it generated which inturn made it hard for the driver to turn the car...
Please suggest me some alternative way to stop this movement...

[Chris Craig]

If your meaning reduce the amount the rack travels, a couple of slotted/normal nylon washers on both sides as it exits the rack housing will reduce the travel and give you a tiny amount of friction if any

[EHog]

you're going to have to clarify or upload pictures if you want help. Myself and Chris Craig appear to be confused.

[DEAD_MAN]

Its not the lateral travel movement thats the main problem.
Its the rotational movement ie rotation about the rack central axis..ill try an upload any pic that i can get to show u...

[SNasello]

Sounds like you have a problem with free-play in the rack, ie the centre distance of the rack and pinion gear are not exactly what they should be.

The only solution I can see is machining a new housing that allows you to adjust the centre distance.

[BillCobb]

It's called 'rack roll' and can be cyclic with the pinion rotation. You need a good flat on the rack and an adjustable slipper bearing to help with this. Use teflon (TM) on the slipper shoe. Examine the rack components of a typical FWD passenger car. Take one apart and get an eye full.

[Pico]

t the lateral travel movement thats the main problem.
Its the rotational movement ie rotation about the rack central axis..ill try an upload any p

Here, this is a good reference:

http://dsr.racer.net/images/pa...eering_rack_assy.jpg (http://dsr.racer.net/images/parts/914_steering_rack_assy.jpg)

If you're getting a lot of rack roll you must have a good bit of play, if I'm not mistaken?

[Mehul Botadra]

Yes we're using a similar system. The problem is not the play, the problem is that the pinion gear gets stuck and the driver has to apply more effort. We need to hold the rack in a certain position with milling any slot on the lower end.

[DEAD_MAN]

@Billcobb: ya exactly...the rack roll is the problem we are facing.
Would definately look into the adjustable slipper bearing

[BillCobb]

This phenomenon is caused by the directional contact forces at the surface of tooth interaction. If the pinion is perpendicular to the rack and the gears are straight cut, there should be no roll. If the pinion has an off axis included angle, some roll may be noticed. A REALLY good pinion machining job and slippery rack teeth can eliminate this as a problem. There is even special grease made to lessen it.

It is still observed as lost motion in a dynamic sense and shows up as hysteresis in a C-factor test. C-Factor is the ratio of rack displacement to pinion rotation, usually expressed per 360 degrees (1 complete revolution) of the pinion when the rack is centered in the housing. Obviously this can be a function of rack position when racks have variable tooth spacing or a humped outline, in 2 cases of a variable ratio gear. Rack and pinion separation forces hac be very high, hence the need for a spring loaded slipper bearing to keep them engaged. Too much force and it becomes a returnability problem.

As a side note, if you would ever consider power assisted steering, the powered rack is equivalent to a power transistor. You put in a little bit of force and get a lot of force out. As you would expect (as engineers of course), such a system can be very unstable in closed loop control. The rack friction is your friend in this case and very necessary. Otherwise all kinds of self generating steering oscillation ("shudder") will form.

When your cars get 1000 lbs of batteries in the next few years, you may run into this...