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What's inside a starter pinion gear assembly, and how exactly does it work?

N

nicklanigan

Contributing Member
Hi all, I've had cause to do a bit of work on my starter motor as part of an overall rebuild, and have discovered a problem with my pinion gear on the starter. The replacement gear is on order, so all I'm looking for is some info on what is inside the pinion gear, and how it works. Like many, I enjoy learning and understanding about these matters.. My symptom to provide context is that the pinion assembly flies up beautifully, engages the flywheel perfectly, but then will struggle to actually turn the flywheel - however, the starter motor is spinning at full speed. The pinion is "slipping" on the starter shaft. The shaft thread and female mating thread are both in perfect condition. Potentially my pinion assembly is 40 years old.

Just to be clear - I'm talking specifically about the pinion gear assembly - I had always assumed this is one part, but upon removal, it is actually at least 3 components - I don't the names for any of them, so hopefully I can make it clear enough... there is the actual pinion gear that engages the flywheel, this is on the top. On the bottom, there is a female "nut" that has the female helical thread that mates with the helical thread on the starter shaft. And there is something between these two components - some kind of rubber bushing or a spring perhaps? I can't see an easy way to get the pinion and bottom nut apart to see what's inside. On my assembly, I'm able to easily turn the pinion one way and the female nut the other way - I imagine this is not meant to happen and is the slipping I'm seeing when using the starter.

So, I know that the female thread is the mechanism by which the pinion assembly is thrown up in the flywheel upon spinning the starter, and is the same mechanism that the pinion assembly descends on once the motor is started or power is killed to the starter.

What happens when the pinion is actually engaged with the flywheel though - presumably the female nut thread must stay engaged with the starter shaft thread, and transfer the torque to the pinion via the rubber bushing/spring in the middle of the pinion assembly? What is the purpose of the spring/rubber bushing - why not just have a single piece pinion gear that has a female thread on the inside? Is it to provide some slippage when a high speed starter engages with a stationary flywheel to avoid the starter stalling out? Impact absorption?

Rather a long question, but hopefully it makes sense.

Thanks,

Nick.
 
Sounds like the drive gear separated from the worm drive. When the starter starts to spin the worm lifts the drive gear into the flywheel and as soon as the engine fires it spins the drive gear down and the spring holds it down.
 
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I reckon the newer ones do have some sort of shock absorbing hub. Why they do, I don't know. They worked fine for many, many years without the feature. Must be an "improvement".
 
Nick.... Keeping it simple, the bendix gear assembly contains a inner clutch to absorb shock as the gear engages the flywheel teeth. It is such a tight fitting assembly that the outer portion (teeth) and the inner portion (spiral gear) appear to be one piece.

However, if the clutch fails (as yours has) the spiral inner portion turns within the outer portion. The cure is simply to replace it.
 
Thanks Joe - looks like my thinking was broadly on the right track. I'm in New Zealand - parts (at a reasonable price) need to come from the USA so we get time to think about failures. I typically have a two week wait for new parts.

Johnny - the interesting thing is that this problem only turned up after I did a brush replacement - suddenly the torque output of the starter went through the roof, causing the inner clutch to fail... grrr... but of course it used to struggle to get the motor spinning fast enough.
 
nicklanigan said:
After I did a brush replacement - suddenly the torque output of the starter went through the roof, causing the inner clutch to fail... but of course it used to struggle to get the motor spinning fast enough.
Click to expand...

Yes, understood. Everything/anything will fail eventually... however to have a component fail right after one has finished some other kind of repair will have the thought going through ones mind, such as....... "What the Hell did I do now?", when in reality it was/is just a coincidence... Been there/done that!

Waiting for parts?.... Me, Central Florida USA (Tampa area)... Not as long a wait as you but I have all of my yard equipment apart in various stages... Riding mower, walk behind self propelled mower, weed whacker, chainsaw, etc. The waiting game drives me nuts!
 
Joe, why don't you just hire somebody to do your lawn work and then you wouldn't have that problem ?? :cool:
 
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Boobie said:
Joe, why don't you just hire somebody to do your lawn work and then you wouldn't have that problem ?? :cool:
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Yeah, of course you're right Boobie. However I just need to keep my hand in so to speak, keep as active as possible. You know what happens when you buy that rocking chair. When I go, I want to be on that riding mower with the wheels aimed straight at that huge flower bed of the one neighbor I can't stand! :)
 
Just closing this out - new pinion arrived - in mine the rubber bushing inside had perished - new one is effectively one solid unit... all works great.
 
nicklanigan said:
Just closing this out - new pinion arrived - in mine the rubber bushing inside had perished - new one is effectively one solid unit... all works great.
Click to expand...

Great... We appreciate the update.

Just in case you haven't already... Install the nut on that bendix drive with a air impact wrench (difficult to tighten otherwise), using Loc-Tite on the threads. You don't want that thing coming loose!
 
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