Lead screw shear pin

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Larry$

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I think most lathes probably have a shear pin to protect the gears that drive the lead screw. Most likely brass. When I got my 1440 I went over it with a fine tooth comb and discovered a steel roll pin in the lead screw. Thinking that seemed like a bad idea I looked it up in the parts diagram. There it showed a brass shear pin. I made one and replaced the roll pin. I should have put punch marks on each side so if it does get sheared I'll know when it is lined up to be able to push it through. Will do tomorrow. This information is courtesy of
 
Larry$ said:
I think most lathes probably have a shear pin to protect the gears that drive the lead screw. Most likely brass. When I got my 1440 I went over it with a fine tooth comb and discovered a steel roll pin in the lead screw. Thinking that seemed like a bad idea I looked it up in the parts diagram. There it showed a brass shear pin. I made one and replaced the roll pin. I should have put punch marks on each side so if it does get sheared I'll know when it is lined up to be able to push it through. Will do tomorrow. This information is courtesy of
Click to expand...

Thanks, I'm in the process of replacing my leadscrew and hadn't even thought of this, the stock ones on my Bolton are steel so I will turn some brass ones.

John
 
Nearly all industrial machines, even including South Bend lathes do not use brass pins. I have never sheared any type of pin on any lead screw, if one has had to replace hundreds of pins, something is being done wrong.
 
The video is from a school, not that anything is ever done wrong in such places. The best safety device is one that is never used. Given the difference in cost of replacing a brass pin or some gears..... My lathe is a PM1440HD and shows the pin as brass, seems reasonable to me but I'm no expert at anything. The feed shaft is also protected but with spring loaded ball detents. Again, seems reasonable. Over the years I've had a lot of employees that were less than perfect machine operators. Actually, I can't remember a perfect one. I had never thought of using South Bend as a reference standard. If the site moderators find this thread objectionable they should let me know & remove it. I thought I was being helpful, sorry I offended.
 
Larry$ said:
The feed shaft is also protected but with spring loaded ball detents.
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My Asian import has a spring loaded ball detent and it was the ONLY thing I didn't check. I was watching closely on an awkward turning job working on the face plate on auto-feed and the apron bottomed on the leadscrew cover. Broke three teeth on the feed pinion.
Turns out the retaining/adjusting screws for the balls had been cranked down hard bottoming the springs. I think I might re-visit the area and look at the shear strength difference, if any, between the tension pin and brass. Thanks for the reminder.
 
NortonDommi said:
My Asian import has a spring loaded ball detent and it was the ONLY thing I didn't check. I was watching closely on an awkward turning job working on the face plate on auto-feed and the apron bottomed on the leadscrew cover. Broke three teeth on the feed pinion.
Turns out the retaining/adjusting screws for the balls had been cranked down hard bottoming the springs. I think I might re-visit the area and look at the shear strength difference, if any, between the tension pin and brass. Thanks for the reminder.
Click to expand...
The balls on mine had also been tightened all the way so it wouldn't have worked. I discovered that when I had the shaft out for another reason. Now adjusted so it will work.
 
When I sold snowblowers I kept a stock of shear pins for the augers. Lots of folks replaced them with regular bolts as these pins are steel and look just like a normal fastener. A frozen newspaper or random rock can do some serious damage to the augers or gearboxes in a hurry.

John
 
The leadscrew shear pin on my Emco Super 11CD is made of aluminum, not steel or brass. I've done a lot of threading on this lathe and have yet to snap a pin but if I do, I'll use aluminum. I think the alignment marks are a good idea, though.
 
My Causing 5418 has a brass pin. I sheared one once because I over-tightened the far end of the lead screw at the tail stock end pillow block. Engaged the power feed and sheared it. Statement of the obvious, the pin needs to shear prior to breaking off gear teeth if you crash or get too aggressive. A steel pin would be fine assuming the design loads were considered.

Bruce
 
My Sheldon MW-56-P has a steel pin in the lead screw. However it has a phenolic gear in the lead screw drive train. It's supposed to "Quite the drive train, and provide protection". Other similar units use a bronze gear in the same position presumably for the same reasons.
 
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