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Author Topic: Help: Steering Gear Specs and Sources  (Read 849 times)
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Help: Steering Gear Specs and Sources
I have a mobility scooter that I want to steer via an Arduino. I need to establish the specs for a proper gearing system to control the direction of the nose wheel. It would help to find sources that sell inexpensive gears - spur gears and worm gears, perhaps - some source with alot of choices as to size and hub diameters, etc.

I have attached a picture. The shaft is almost 1" in diameter; the rake is about 30 degrees from vertical.
Normally, there is a tiller attached to the steering shaft - at the point of the masking tape and bolts.


* TillerBase.jpg (374.01 KB, 1600x1200 - viewed 12 times.)
« Last Edit: December 18, 2012, 01:33:04 pm by louarnold » Logged

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How much are you willing to spend?

The quick and easy (but not cheap) method of doing this in a manner where you aren't throwing good money after bad - is to use a linear actuator. You can buy them new from different places (Firgelli, Pololu, Ebay, etc); you will want to look at a few factors like speed stop-to-stop (most are fairly slow - 0.3-0.6 inches per second movement), as well as power, both driving and holding (ie, 150 lb driving, 400 lb holding). Also look into stroke length (actually, you might want to figure out the arc and needed stroke length for your application first). Some can be backdriven, some can't (whether you want that or not is up to you).

You can also find used ones as well (some powerchairs use them for tilting the seat, for example - and you find these often online), but they still might cost you a bit. Whatever kind you look at, most are meant to be driven from 12-24 VDC by an h-bridge. Some have end-stop switches built in (shutting off power at the end of travel); some have different means of position tracking (potentiometers are popular, but you will typically find some form of pulse switch system - either magnetic or hall-effect; for such a system, you would drive the actuator to one extreme until the switch -stops- pulsing, then you drive it in the opposite direction counting the pulses until it stops pulsing, then drive it in the opposite direction again until it stops pulsing - then take the average of your two readings - half of this number of pulses should equal the mid-point of travel for the actuator).

So - with an h-bridge and your position control method, you basically have to create your own servo system (not as difficult as it sounds, until you start to get into a PID system - but a simple window-comparator servo system on something slower like this will work fine); of course, you're going to have to do this using whatever system you choose, unless you spend the money on an actuator or control board that accepts servo control signals. Note that if you are unable to fully drive the linear actuator to its stops (maybe its length of travel is longer than you need), then you'll want to install and hook up your own set of limit switches. These are very important; I once ruined an expensive gear motor because I didn't have limit switches on a steering mechanism, overdrove the limits, and broke the metal gears inside the motor.

Which brings us to gear motors and servos: If you have to go this route, choose motors that don't use spur gears. In a high-torque situation, it is very easy to break the teeth off the gears, and destroy the gearbox (which may or may not be easy to repair - or if so, may or may not be inexpensive!). In the situation I described above, I attempted to get replacement gears for the motor, but the sales rep would only sell me 50 pcs minimum - at $7.00 per each gear! It would've been cheaper to buy a new motor (which I ended up doing). Most hobby servos use spur gears, even so-called high-torque ones (and for the really expensive servos of this nature, it might be cheaper to purchase a linear actuator). If you can find a gear motor that uses helical gearing, or herringbone gearing (both very expensive, I might add), that would be better. Or - if you can find it, go for a gear motor that uses a worm gear to a spur gear (a windshield wiper motor or a window actuator motor from a car are low cost examples).

For my situation (which I am still working on), I ended up building a homebrew linear actuator mechanism using a 7.2 VDC cordless drill motor/gearbox with a piece of all-thread as my lead screw. A couple of nuts with a bolt JB-Welded to it form the travelling nut (if the JB-Weld breaks, then I'll try a real weld job instead). It should be a cheap (the drill I got at Goodwill for a couple of bucks; the rest I spent less than 10 bucks on) but powerful form of a linear actuator, as well as having much faster movement than a standard linear actuator (1-2 inches per second seems reasonably possible from my testing).
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Windshield wiper motors modified to act as a servo seems to be popular with stearing setups.








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How much are you willing to spend?
"...choose motors that don't use spur gears. In a high-torque situation, it is very easy to break the teeth off the gears, and destroy the gearbox (which may or may not be easy to repair - or if so, may or may not be inexpensive!). ..."

Yes, I was worried about this. What are the typical specs for torque on gears?

"...it might be cheaper to purchase a linear actuator)....
I don't know what it is, but I'll look for one.

"...if you can find it, go for a gear motor that uses a worm gear to a spur gear (a windshield wiper motor or a window actuator motor from a car are low cost examples).
In fact, I thought of a spur gear and worm gear, but the teeth may break, and the shaft is raked back from the vertical so the worm gear on the chassis and the worm gear on the motor will not be in-line.

For my situation (which I am still working on), I ended up building a homebrew linear actuator mechanism using a 7.2 VDC cordless drill motor/gearbox with a piece of all-thread as my lead screw. A couple of nuts with a bolt JB-Welded to it form the travelling nut (if the JB-Weld breaks, then I'll try a real weld job instead). It should be a cheap (the drill I got at Goodwill for a couple of bucks; the rest I spent less than 10 bucks on) but powerful form of a linear actuator, as well as having much faster movement than a standard linear actuator (1-2 inches per second seems reasonably possible from my testing).
I have such a drill motor and I have yet to check the gears, but again - how to mount them?
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Windshield wiper motors modified to act as a servo seems to be popular with stearing setups.
Some interesting videos. Seems like the motors are fast enough and have reasonable torque. How to mount them though?
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