Material for a planet gear carrier in an epicyclic gearbox

[Mike DonnerstagParticipant @mikedonnerstag]

I have a jeweller’s rolling mill with an epicyclic gearbox. However, the cast iron planet gear carrier (link: https://durston.com/product/cast-iron-carrier-for-gearbox-agile-110-drm-100/) is shattered. I was wondering why this would be made from cast iron and whether it would be worth me making one from steel?

The reason I ask is that I recently purchased the machine second-hand and I don’t know whether there are any other parts of the machine that need to be replaced, as these very quickly exceed the cost of a new machine. I’d be very interested in whether anyone thinks it would be worth making the part in steel so that I can at least test that the rest of the machine is good before purchasing expensive parts.

Looking forward to your wisdom,

Mike

[BazyleParticipant @bazyle]

It was only made form CI as that gave the shape easily and cheaply. Sometimes CI is used because it makes a good bearing but not necessary here. Look at the remains you have and see if there are wear marks on the CI surface from the pinions. If so provide washers as appropriate.

[DC31kParticipant @dc31k]

As above, for that shape, cast iron is cheap in production quantities.

Have a good look at where it fits as a simple circular shape is a lot less work to make. Look at its function; do not slavishly copy its form.

If it does need to be that tri-lobed shape, maybe a laser or plasma cut blank would be a good place to start. Make a dimensioned pencil sketch of it and someone would turn it into a dxf file that could be sent to a cutter. All you would have to do is drill and ream for the three pins and bore and keyway the centre.

[Michael GilliganParticipant @michaelgilligan61133]

I can see no need to use cast iron for a replica of that part … anything that locates/maintains the three pins in their required positions is surely fit for the purpose.

MichaelG.

[FulmenParticipant @fulmen]

I agree, mild steel should work fine.

[Ian PParticipant @ianp]

If you have the facilities to make one out of steel, go for it, don’t forget the keyway though.

Steel is a far more suitable material for this part which has to transmit considerable torque. A solid disk rather than any fancy shape would be easier to make. Probably bad design siting the keyway adjacent to a pin (hole) as I suspect that is where the shattering originated.

Ian P

[DiogenesParticipant @diogenes]

..looks a lot like the 3 pin-holes and three longer slots could be on a common radius – visit to the rotary-table and a bit of cutting away & rounding-off.

Can/will you re-use the original pins? ..they might need some consideration if not..

 

[David George 1Participant @davidgeorge1]

I have recently used my rolling mill, ( a cheeper version with a simple gear box) for rolling a piece of gold for a friend, to form a section for a ring and I was suprised by the force needed to roll the gold which needed anealing after each pass as the gold work hardened so easy. I have until this time only used silver which is so much easier to form.

David

[Ian P]

On Ian P Said:

[…] Steel is a far more suitable material for this part which has to transmit considerable torque. A solid disk rather than any fancy shape would be easier to make. Probably bad design siting the keyway adjacent to a pin (hole) as I suspect that is where the shattering originated.

Agreed, Ian

… In fact, although I have no evidence at all for this: I suspect that [historically] the component might have been drop-forged
”continuous product improvement” has a lot to answer-for

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Please forgive the digression

Here’s a good demonstration of the new-ish ‘Olivia’ Mill, featuring a 50:1 gearbox

https://youtu.be/dNRgOtM9jXU?feature=shared

MichaelG.

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Edit: __ with some lovely drawings in the patent

https://worldwide.espacenet.com/patent/search?q=pn%3DUS2023271237A1

[duncan webster 1Participant @duncanwebster1]

Drop forged cast iron??

[Michael Gilligan]

On Michael Gilligan Said:

On Ian P Said:

[…] Steel is a far more suitable material for this part which has to transmit considerable torque. A solid disk rather than any fancy shape would be easier to make. Probably bad design siting the keyway adjacent to a pin (hole) as I suspect that is where the shattering originated.

Agreed, Ian

… In fact, although I have no evidence at all for this: I suspect that [historically] the component might have been drop-forged
”continuous product improvement” has a lot to answer-for

MichaelG.

Like Ian and Michael, I’d have thought steel was a better choice for this component.   Here it is for those who hate clicking on links:

 

Something odd going on!  Seems to transmit power from a keyed motor shaft into a gearbox through the 3 pins.  Is cast-iron a good choice?

For: cast-iron is cheap and easily moulded and machined.  Not much work needed to make this part’s curvy outline, or to bore the hole, cut the keyway, and flatten the base.   Has useful sound deadening properties, not ringing like steel when vibrated:  reducing gearbox and motor noise may be the main reason for the designer choosing cast-iron,  That most cast-irons are self-lubricating, making it a moderately good bearing material, doesn’t seem to be relevant here.

Against:  brittle, weak in tension (roughly 10% of the compressive strength).  Not a disadvantage when this part is driving a constant load because the pins and keyway are all in compression.   But liable to break if the machine is subjected to a shock-load such as a jamb, resulting in the part being tensioned as the forces are transmitted back through the drive train and into the motor.   Possible that cast-iron was chosen deliberately so a relatively easy to replace component breaks rather than something more expensive.   Anyone smart enough to model the part in CAD and do a Finite Element Analysis?   (I don’t have the skills needed to get FEM right or to understand the results!)

Deliberately weaker components designed to break first are common in machinery:  shear pins, plastic gears,  belts, abd fuses etc.

Be that as it may:

1. The safe option is to replace the part with a manufactured spare.  (That these are available off-the-shelf suggests they often have to be replaced.) OR to DIY one out of cast-iron.
2. Though it might be noisy, a steel replacement will work just as well, other than it won’t break if the machine jambs.  Something else will take the shock instead…

Mike’s concern that there may be other damage is valid.  The big problem buying second-hand is not knowing the machine’s history.  I suspect a previous owner did Mike’s roller a nasty in the past, perhaps overloading it repeatedly.     As the whole roller may be ‘Beyond Economic Repair’, i.e. cheaper to buy a new one, I’d be tempted to replace the part as cheaply as possible by making it from mild-steel*.  Then I’d take extra care not to overload the machine in the workshop.  There’s a good chance the repair will last long enough to be worth doing, even if an accident or slowly growing cracks eventually write-off the machine.

* Mild-steel because it’s available!  And even if I had a suitable lump of cast-iron in stock I dislike machining it because of the appalling mess.   Big parts are far worse than small.  I assume this part is about ⌀80mm and 20mm deep?

Dave

[Michael GilliganParticipant @michaelgilligan61133]

If you watch the video that I posted, you will briefly see a fairly big bloke using both arms and substantial effort to drive the mill … methinks it might only be a small step-up from that to ‘explode’ a cast item of this design.

MichaelG.

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P.S. __ we had a good ‘Finite Element’ man on the forum for a while … but he left when the ‘pragmatic’ members started jeering.

[Ian PParticipant @ianp]

Maybe we will get some more information from Mike but I feel there must be more to this particular gearbox than we can surmise from one picture.

If this is a single stage epicyclic reduction then its input power would be a manually rotated pinion meshing inside the three pinions on the stub shafts, with power out to the rolls via a shaft in the keyed hole. To me, something does not look right, the output shaft diameter and its key do not look man enough if the roll pressure and forces involved are anything like, or proportional to, those shown in the video Michael linked to.

Ian P

 

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