Sacrificial pin

[Bernard StartParticipant @bernardstart12493]

Sacrificial or shear pin.

Hi everybody I have made a stupid mistake on my Amabl250Fx550 lathe.

I have driven the saddle under power into the head stock. Ouch!!

The leadscrew pin has sheared and I hope no other damage.

The debris looks like the pin was brass 3mm dia. 21mm long.

Any comment Please before I make a new one.

Is it any old brass or are there any special methods or precautions.

Thanks and best of these times to everybody.

Bernard

[noel shelleyParticipant @noelshelley55608]

Try magnet on the wreckage. If nothing happens, and it looks like brass use 3mm brass rod. If it breaks too  easily then try one of the stronger copper alloys. Noel.

[SillyOldDufferModerator @sillyoldduffer]

I used any old Brass, and it’s fine.  You don’t want anything too strong because, in the event of a crash, shear pins have to break before the machine is damaged.  Steel is asking for trouble.

Also, avoid soft metals like Aluminium alloys.  Their problem isn’t lack of strength, it’s that they tend to distort and stick in the hole when the remains are knocked out so the new pin can be fitted.

Isn’t it wonderful that a lathe can make the parts needed to fix itself!

Dave

[Harry WilkesParticipant @harrywilkes58467]

Dave in a previous life I had the pleasure of keeping an old Binks chain on edge conveyor going it was old and worn out (just like me). When we loaded it with the heavier brake discs we sprayed it would often take the shear pin out so when we turned up a new one out of brass we would do around 20 and hang them up in a bag next to the drive 🙂

H

[Howard LewisParticipant @howardlewis46836]

Having done something similar, which bent the drive shaft and the integral gear, found that the Leadscrew and Feedshaft were secured with rollpins, by the factory!

(It started life with a 2 hp single phase motor but was fitted from new with a VFD and 1.5 hp three phase motor).

They were replaced by 5 mm brass shear pins, with a 2.5 mm hole drilled through, to weaken them.

Howard

[Martin KyteParticipant @martinkyte99762]

Off topic a bit but old farm machinery like binders used to use wood as shear pin material. The big advantage was that miles from anywhere a new one could be cut from any handy hedge.

[SillyOldDuffer]

On SillyOldDuffer Said:

I used any old Brass, and it’s fine.  You don’t want anything too strong because, in the event of a crash, shear pins have to break before the machine is damaged.  Steel is asking for trouble.

Also, avoid soft metals like Aluminium alloys.  Their problem isn’t lack of strength, it’s that they tend to distort and stick in the hole when the remains are knocked out so the new pin can be fitted.

Isn’t it wonderful that a lathe can make the parts needed to fix itself!

Dave

Well I would disagree with you here Dave, Emco lathes (Super 11) are fitted with an alloy shear pin on the feed shaft from the factory,  I made some replacements from alloy rivets, no idea of the original spec but it is a soft alloy thus me choosing rivets as the material.  Anything stronger would be inappropriate and probably cause damage.  Its better too weak than too strong !

John

 

 

[SillyOldDufferModerator @sillyoldduffer]

On 15 December 2024 at 18:54 JohnF Said:
On 14 December 2024 at 18:18 SillyOldDuffer Said:
…

Also, avoid soft metals like Aluminium alloys.  Their problem isn’t lack of strength, it’s that they tend to distort and stick in the hole …

Dave
Well I would disagree with you here Dave, Emco lathes (Super 11) are fitted with an alloy shear pin on the feed shaft from the factory,  I made some replacements from alloy rivets, no idea of the original spec but it is a soft alloy thus me choosing rivets as the material.  Anything stronger would be inappropriate and probably cause damage.  Its better too weak than too strong !

John

 

Agreed it’s better for shear pins to be too weak rather than too strong!  But, as is common in engineering, the optimum is some sort of balance, here between a pin that won’t break when the lathe is working normally (a pain in the proverbial), and a strong pin that doesn’t break at all.

We’re both at a disadvantage compared with EMCO’s designer.  He will have known pretty much everything about the machine: power, materials, normal loads,  breaking stresses etc, plus experience of lathes and how they wear and break in practice.   Makes it possible to choose the diameter and material needed to shear at a known load.   The rest of us have to guess!

Selecting rivet metal for shear pins on the basis that EMCO used an unknown alloy on one of their lathes feels a bit of a stretch to me.  But John’s guess is as good as mine!   I pointed out though that metals that expand into holes when hammered won’t be as easy to remove as a harder metal.

Also beware assuming a metal’s malleability with weakness.   Rivets can be tough – depends on what they are made of, and their intended purpose.

Brass, I think, is a safe compromise.   Weaker than steel, unbending enough to punch out cleanly, and easy to drill out if it comes to that.  Common too: “any old Brass” is much easier for Bernard to source, compared with “an unknown alloy rivet of the type John chose”.

I don’t think shear pins on Far Eastern hobby lathes are fussy though.  Just avoid steel, especially tough guys like roll-pins and dowels!

Dave

 

[noel shelleyParticipant @noelshelley55608]

IF you have a briggs and Stratton engine, the key for the flywheel is alloy. The idea being that if it’s on a lawn mower of the rotary type then hitting a brick will cut the shear key not break the crankshaft as it stops suddenly. This key also sets the timing and even the slightest step or partial shear will stop it from starting. Had it happen. A new one can be filed from a bit of ally scrap.  Noel.

[Bo’sunParticipant @bosun58570]

Hello Bernard,

Agree with others that brass is your best bet, but before setting about making a new one, check whether it’s a tapered pin.  It was on my Warco lathe, which may well have come from the same source.

[Harry Wilkes]

On Harry Wilkes Said:

Dave in a previous life I had the pleasure of keeping an old Binks chain on edge conveyor going it was old and worn out (just like me). When we loaded it with the heavier brake discs we sprayed it would often take the shear pin out so when we turned up a new one out of brass we would do around 20 and hang them up in a bag next to the drive 🙂

H

And a very good idea too!

Wear in the machine means it starts and stops with a hefty clunk.   Makes sense to absorb clunk energy by breaking a sacrificial pin rather than using it to increase the wear, causing even bigger clunks in future. So tempting when shear pins break in normal operation to fix the obvious instead of thinking it through.  Bet there was at least one person utterly convinced the correct answer was to fit a stronger pin.  If so he focussed on the wrong problem; it’s not how to stop pins  breaking, it was how to extend the life of a worn machine.  My guess is that bag of spare pins saved a lot of money.

Have to be wary of keeping old machines going though, because firm that get into the habit are liable to end up competing with enterprises running modern machines that leave older equipment in the dust, even if the old kit is still 100% reliable.   There’s a point after which keeping old tools going by patch and mend becomes a deadly mistake.   The mistake may never become obvious on the shop-floor, but it is to the shareholders.  Leaving it too long, productivity falls continuously relative to up-to-date competitors, the firm loses financial credibility, can’t raise the cash needed to modernise, and then eventually goes bankrupt.

Dave

 

[old martParticipant @oldmart]

How much stress did it take for the pin to shear? If you would like less, then reduce the diameter slightly at the point where it is made to break. The Smart & Brown model A has a shear pin in the input gear of the change wheels. Made of steel about 2mm diameter with a section near the centre turned down to 1/16″ diameter by 1/16″ long. S & B neglect to mention the grade, so I made several spares out of mild steel, and we haven’t broken any of them yet. When I doubled the output speed to cut a 6tpi, I put a temporary plain pin in and was very careful not to have a crash.

I would try bending the remaining bit of brass to judge its strength, ordinary drawn brass rod would be the best bet.

[Mick BaileyParticipant @mickbailey28509]

Not every shear pin is soft. I still have some shear pins from a 1930s Crabtree press and they’re made from hardened steel 3/8″ diameter. You can’t touch them with a file and over the years they’ve been a good material to grind cutters from. When the original supplier ceased we had them tested and made locally but it was initially difficult to get the correct specification and a lot of trial and error.

The pins snapped cleanly with no pre-failure creep. Due to the massive torque they also needed to transmit a lot of power in normal use.

[Neil WyattModerator @neilwyatt]

Brings back memories of my first job… much of which involved using Tirfor winches to pull HUGE rhododendron root plates out of the ground with regular shear pin replacement.

We then chainsawed them up, luckily the ground was peat so no hidden pebbles.

[Mick Bailey]

On Mick Bailey Said:

Not every shear pin is soft. I still have some shear pins from a 1930s Crabtree press and they’re made from hardened steel […]

The pins snapped cleanly with no pre-failure creep. […]

That all makes very good sense, Mick

MichaelG.

.

I suppose the soft variety would be better-named ‘smear pins’ 🙂

[Martin Johnson 1Participant @martinjohnson1]

We used to fit one of our plunger sludge pump ranges with shear pins –  soft steel (a section of 6″ nail on the shop parts list) contained in hardened bushes in the drive and driven side.

One site reported lots of problems with sheared pins, so to avoid further trouble they had taken out the bushes and put a solid steel rod in.   Doh………..

Martin

[old martParticipant @oldmart]

No longer a shear pin, but a nice drive pin. The Smart & Brown shear pin runs inside hardened bushes.

[Howard LewisParticipant @howardlewis46836]

The purpose of a shear pin is surely to protect other parts of the drive train from damage, by failing before anything else suffers. It should be easily accessible, and replaceable.

I suspect that some of the newer lathes, and mills use plastic gears for the same reason.
better to break something that is easy and cheap to replace rather than some expensive item, such as the control board!

Howard

[mark costello 1Participant @markcostello1]

A machine that started acting up had a “V” shaped groove in it’s shear pin. To get it to run until the next planned (Ha Ha) maintenance session the groove design was changed to a “U” shape and it worked.

[old martParticipant @oldmart]

Rod 3D printed a pair of gears for the tumbler reversing lever on the museum’s Atlas lathe as I was worried about the teeth on the cheap Zamak, (Mazak in the UK) gears which worried me, as this model has no shear pin in the drive train. Of course, the plastic gears might prove stronger than Zinc.

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