I have been asked to make a re-designed blades and shaft for a cremulator (do you really want to know what that is? ) as the current system regularly suffers from failed bearings on a 6 monthly basis. The current design has a 1.2kW motor driving a vertical shaft and supported by a flanged self aligning bearing outside of a drum, inside the drum the shaft is supported by a flanged oilite bush which quickly degrades with the abrasive nature of the contents. The blade bosses are attached via a 4mm keyway and grub screws.

I have therefore upgraded the blade shaft from 13mm to a 20mm shaft and changed the design to a square form for the blade locations. Blade bosses will be laser cut to give a square internal form, grub screws are deleted and the whole assembly is clamped by a M16 threaded nut on the shaft end. Under the bottom boss is a 8042RS sealed ball race in stainless within an alloy housing clamped to the drum base and a suitable 'O' ring sealing under this.

I ordered some 20mm BDMS and proceeded to cut to length and centre the ends prior to machining a keyway for the Fenner bottom drive adaptor (this is currently 20mm dia in the original design so making the whole shaft 20mm seemed a good idea and saved on the machining required ). When I came to cut the keyway I found the shaft was bent, making the square section an odd shape! as the flats of the square do not fully meet their adjacent flats. I then checked the bar and found it was bowed by 0.2mm over 140mm length (0.008" over 5 1/2" ). Picking up the rest of the bar it was twisted and bent – I took the piece back to the metal suppliers and asked for a decent bit of 20mm BDMS that wasn't like a dog's back leg, they had to dig out some old stock that was something like acceptable. The boss was muttering about the quality of the material they had been receiving lately and showed me a length of Aerospace specification alloy bar which had visible inclusions on the surface, and showed me a internal void exposed where they had cut the material for a RR order.

What is the generally accepted allowance for straightness in BDMS? The diameter was pretty low at 0.04mm undersize, the old stock was much better at 0.01mm under.

Two guesses at where the material originated!

Martin

Thanks for all the replies, and yes I guess it was in tolerance according to Jason's data. Although the bend was 0.2mm (0.008" ) in the centre of a piece 140mm long (i.e both ends touching the surface plate and an 0.008" feeler gauge inserted beneath in the centre )

Although the machine is not what you would describe as a 'precision' piece of engineering it has run for a number of years on the original design (which, to be honest, I never liked in the first place as the shaft was effectively overhung too far from the flanged self aligning bearing at the base of the drum – the oilite bearing was pitiful ). I wanted to support both extremities of the shaft but in this small machine it is difficult to achieve – a much larger machine with multiple contra rotating blades on 3 shafts has worked from day one without any trouble. These shafts are end supported. Guess next time I upgrade another I'll have to go for 22mm (7/8th) and turn it down for its whole length or get some centreless ground bar.

The 'cremulator' by the way is a machine for reducing the rather large 'clinker' left over post cremation into a neat pile of dust for storage in a jar on the mantlepiece! Horses are the most difficult needing the big ones, the small ones are for Pussies and Rover. Humans come in between.

Martin

Edited By Oily Rag on 14/10/2021 21:33:07