# Using a slot drive motor



## electro wrks (Mar 5, 2012)

What failed in the original set up?


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## JRP3 (Mar 7, 2008)

The tang on the end of the shaft that engaged with the slot. Our theory is that the single bearing created a pivot point for the shaft under load, which eventually fatigued the tang and cause it to shear off.


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## electro wrks (Mar 5, 2012)

It's tough to line up more than 2 bearings on the same rigid shaft. Better to have 2 bearings on the adapter (like you have now), and a little bit of play in the slot( a few .001 " ). This would allow the slot and tang to act like a basic flex coupling to account for minor misalignment. Also, I would have machined the step on the motor end housing to fit the OD of the adapter flange. The way it's set up now, it looks to me, an over-tightened adapter mounting bolt (or loose one) would pull the adapter out of alignment.

This looks to be a pretty small shaft, compared to the rest of the motor. Overall power, reversing rotation, and shock loads might be limited by the small size of the shaft and tang.


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## major (Apr 4, 2008)

It is a power steering pump motor from a forklift. It is compound wound therefore unidirectional. Made to run continuously at no load and loaded with pump when steering required. Typical rating about 3/4 hp and max load limited hydraulically to about 2.5X at pump by-pass (end of steering throw).

Slot/tang drives were always a bitch. Needs almost perfect alignment and lube. I still dislike slot/tang even on a screwdriver


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## Duncan (Dec 8, 2008)

major said:


> It is a power steering pump motor from a forklift. It is compound wound therefore unidirectional. Made to run continuously at no load and loaded with pump when steering required. Typical rating about 3/4 hp and max load limited hydraulically to about 2.5X at pump by-pass (end of steering throw).
> 
> Slot/tang drives were always a bitch. Needs almost perfect alignment and lube. I still dislike slot/tang even on a screwdriver


I will add to that slot/tang drives DO NOT LIKE any torque reversals - not even the small one when overrunning your motor
Its not so much the torque as the reversal - the tang bashes across its clearance 

In the pump the load is always one way


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## JRP3 (Mar 7, 2008)

electro wrks said:


> Also, I would have machined the step on the motor end housing to fit the OD of the adapter flange. The way it's set up now, it looks to me, an over-tightened adapter mounting bolt (or loose one) would pull the adapter out of alignment.


Actually if you look closely at the third picture you'll see a bright aluminum ring on the motor. This spacer fits into the bore of the adapter base to locate it.



> This looks to be a pretty small shaft, compared to the rest of the motor.


Since the weak point is the size of the motor slot using a larger shaft would not improve the situation. It's a shame really, the shaft inside the motor is quite a bit larger, but they machined it down to a much smaller diameter, and cut a slot in it.


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## JRP3 (Mar 7, 2008)

Duncan said:


> I will add to that slot/tang drives DO NOT LIKE any torque reversals - not even the small one when overrunning your motor
> Its not so much the torque as the reversal - the tang bashes across its clearance
> 
> In the pump the load is always one way



Yeah, probably didn't help that in my original application in the tractor I just used an on/off switch, so even though I'd start it with the clutch in, and under "no load", there was still the inertia of the pulley, plus the unloaded hydraulic pump and belt. Bad idea all around.

Another idea we kicked around was to drill and tap the end of the motor shaft to accept a threaded shaft and eliminate the slot completely.


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## electro wrks (Mar 5, 2012)

Since the weak point is the size of the motor slot using a larger shaft would not improve the situation. It's a shame really said:


> If the output shaft of the motor was larger in diameter, I'm assuming the slot and the tang would be wider and longer (larger) and able to transmit more power with less load and wear on the slot and tang. Bigger is better.


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## JRP3 (Mar 7, 2008)

I thought you meant if the shaft on the adapter I made were larger it would be better. Yes if the motor shaft itself were larger it would certainly be better. A spline would be much better than the slot as well.


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