"Weak link"

[Jim Hubbell]

Crashed my Atlas and broke the lead screw thrust bearing. Made a replacement and it is up and going. My thought is to build a new bearing assy using a shear pin. Don't know how to size it to be in safe shear/thrust load area.
Any ideas/info welcome.

[MikeL46]

My leadscrew shear pin is a 1/8" mild steel pin. Enco 13X40 lathe.

Mike

[brian Rupnow]

I would start with a 1/8" brass pin. That's what my 12 x 18 lathe has. Remember that if the pin doesn't shear, you are going to break a gear or a casting.---Brian

[Benta]

+1 for brass. Mild steel doesn't shear well, it just deforms.

[CalM]

Taken from page 1846 of one of the reference books in my library (By the page count alone, you can tell it is a sizable work ;-)

If the pin and the shaft are make from the same material (equal strength) then if a round pin of 40% of the diameter of the shaft is fitted in a close drilled hole, the pin and shaft will carry the same load.

This has nothing to do with sizing the pin for any other consideration except the pined connection and shaft load capacity.

There is a 3 mm mild steel pin serving as a sheer safety for the input to the Norton Box on my Weiler LZ330. It was broken when I first got the lathe, Nothing else though. So it works!

[old mart]

The shear pin for the Smart & Brown model A is 1/16" dia in the middle and runs at a 3/4" radius. I don't know offhand what the speed/torque is at that point.

[darryl]

Obviously a lead screw has to remain in sync with the gearing, so an adjustable slip clutch would not be preferred- but otherwise I'd consider that. Cordless drills come with adjustable torque settings, which is basically some spring loaded balls in detents. That could be something else to consider as it's positive in engagement but will skip when overloaded. In this case there's nothing to have to replace except a ruined workpiece, etc. Of course anything other than a replacement shear pin will take up more space.

Just throwing in another idea.

[Jim Hubbell]

My thinking so far is to utilize the axial thrust of the lead screw. As did the original. Fasten the bearing to its mount with a pin. Axial over-thrust to shear the pin.
Thanks for the replies. Keep them coming.

[brian Rupnow]

I've been down this exact same road, having a terrible fear of "crashing" my new 12 x 27" lathe under power feed and taking out a gear box. Your options are :a shearpin, but only a brass one, not steel, an aftermarket slip clutch which can be retrofitted to the drive for about $400, or something home-made.

[Circlip]

Shear pin on an eMco Maximat is 1/8"dia 6082 Aluminininium .

Regards Ian.

[GNM109]

My leadscrew shear pin is a 1/8" mild steel pin. Enco 13X40 lathe.

Mike

I have a 13 X 14 Enco geared head that I bought new in 1994. It came with a tool box with two chuck keys, reverse jaws for the original 6" chuck, 3 spare D 1-4 mounting pins, 2 screwdrivers, a wrench for the OEM lantern style tool post, a small can of touch up paint and a half-dozen 3mm aluminum replacement shear pins. When those ran out, I make new ones from 1/8" aluminum welding rod by sanding them down in the lathe with emery cloth.

I guess steel would work if it were very soft, but they should be aluminum so that they will shear at the proper torque. JMHO. .

[firbikrhd1]

I'm not sure how much force it takes to shear a flywheel key for a Briggs and Stratton small engine, say 3 HP, but B&S uses an inexpensive, readily available aluminum key to protect the crankshaft and flywheel and maintain proper timing. The key is designed to shear and shears if the lawnmower blade hits something solid. In my experience that key will shear at fairly low torque if the flywheel isn't tightened securely on the crankshaft taper, which is really what takes most of the load.

What I'm getting at is perhaps a B&S flywheel key fitted to a machined coupler at the headstock end of the lead screw would be sufficiently strong to prevent shearing under normal conditions but shear cleanly in a crash. Since those keys are inexpensive and readily available replacement after an accident would be a snap.

[J Tiers]

It's NOT the shaft strength at all that is the limit.

The leadscrew can put a good deal of force on the halfnuts etc when the screw may not have a lot of torque on it. Principle of the screw jack, after all. So the pin is to protect the OTHER stuff, not just the shafts.....

Brass or aluminum, probably. You could even test, by putting a scale, or load cell if you have one, between carriage and tailstock, and checking force vs torque.... the force you feel that you are uncomfortable with should break the pin.

[John Stevenson]

Early Myford had a two piece leadscrew with a 1/8" brass pin.
Later models had a one piece screw with no shear pin and so they used to strip the fibre tumbler gears.

This was progress.

[CalM]

For every aluminum shear pin, there is a smaller , lighter. cheaper steel pin with the same shear value ;;-)))