Brushless motor shafts

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JohnS

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With my workshop facilities my services are frequently called upon by fellow radio controlled model plane flyers to produce replacement shafts for brushless motors, usually 5mm diameter. Currently I am making these from silver steel rod and am unsure if there is any advantage in hardening and tempering after I have machined the required flats and circlip grooves. I would welcome any guidance on this subject.

John S
 
maybe increased wear resistance, but you may end up warping the shafts.

are these shafts running on ball bearings? if so i wouldn't bother.

if they are the cheap plain bronze bearings hardening could help, but consider why the first one had to be exchanged in the first place. was it worn out, bent, broken?

i'd think that a springy temper would be the best, too brittle would be easier to break during crashes.

silver steel is a fairly stiff material by itself, more than enough for the application i guess.
 
thanks enfieldbullet

Yes the shafts run in ball races and I haven't seen the need to harden them especially as you point out there is the danger of distortion. The motors come to me with bent shafts due to crashes, or 'heavy landings' as flyers prefer to call them. I did wonder if hardening would reduce the likelihood of bending on impact but, on the other hand, it would presumably increase the risk of breaking.

JohnS
 
I have bent a few shafts recently on my outrunners, and found the replacement shafts are quite inexpensive. Since they are usually metric sizes, it is not feasible for me to make replacements.
I wonder if you find it worthwhile to make them, and how do you get metric round stock for them.
Mosey
 
It is a matter of convenience. Stockists of replacement motor shafts are thin on the ground in the UK and with a wide range of different motor manufacturers it is not viable to stock every type. Buying via mail order is time consuming and costly when you add on shipping costs. Most metal stockists carry stock of metric silver steel (drill rod) and a new shaft can be made and installed within a few minutes.

My original question related to the advisability of hardening the shafts not whether it was financially viable. It would appear there is little advantage going thro this procedure.

JohnS
 
if you did temper it to spring tough it would probably be the best thing around.

crashes wouldn't break it or bend it, it would just spring back to shape (unless you go nose first perhaps, on concrete or something)

but that would be for a special friend. that reminds me when i was into bowmaking, people would ask the difference between 2 ply and 3 ply string. the consensus was that 2 ply is what you make for other people, 3 ply is for your own bow.
 
'Spring tough', now that does sound interesting. Can you tell me how I might acheive that condition with my motor shafts ?

JohnS
 
quench while non-magnetic to fully harden (i do it like this, not by color, way more accurate IMO. heat it and test it with a magnet every once in a while, when it doesn't stick it's time). use oil for the quench (it's not as crack inducing as water).

the reason you want the lowest non-magnetic temperature is because the grain will be smaller, and the steel stronger.

then temper it in the oven until an even blue/purple. (indirect heating here is best, take your time, but some oven's wont reach blue temps and you have to figure another method)

maybe a little behind blue (more brownish) for extra strength.
 
Thank you, thank you ! That's just the sort of guidance I was hoping for.

JohnS
 
keep in mind many systems have sacrificial parts. sounds like the shaft is the one here if you make the part that is designed to fail too strong a more expensive harder to replace par will take the blow.
as far as hea treating try a piece of w-1 rod heat it up and dip verticaly in used motor oil.

Tin
 
Try to get 42CrMo4+QT (that is 4140H or 4140RH)
It is quite reasonably to machine, but still quite tough. That material is already quenched and tempered (thus the QT).

I bet, the original shafts are centerless ground. You can't get any good fitting for a ball bearing with a plain vanilla drawn steel. Drillrod or silver steel ain't much better, their tolerance field doesn't match what you need for a ball bearing to fit.

To make a single perfect replacement, is absolutely not economical.


Nick
 
Heat treating properly without a furnace I think would be difficult. I used drill blanks for replacement motor shafts, and some manufacturers do in production. Drill blanks can be problematic when grooves are necessary. Usually, OEM spares (sometimes even from another brand name as many are rebranded at the same factory) are so cheap that it's hardly worth making a shaft.

All that is made in China and sometimes the heat treating on OEM shaft can be terrible. When my employer first started making brushless motor in China in 2003 or so, the shafts were either dead soft or brittle. It took a while to find the right supplier who could consistently supply a tough shaft.

When buying a motor, you really do need to consider spares availability unless you plan to throw it away when you break a shaft.

Greg
 
Try to get 42CrMo4+QT (that is 4140H or 4140RH)
I bet, the original shafts are centerless ground. You can't get any good fitting for a ball bearing with a plain vanilla drawn steel. Drillrod or silver steel ain't much better, their tolerance field doesn't match what you need for a ball bearing to fit.
Nick

for that what i would do is turn the shaft a little oversize, almost wanting to go in, then lap it to fit.

i believe most mini-lathes don't have the required spindle runout required to turn bearing fits anyway.
 
i believe most mini-lathes don't have the required spindle runout required to turn bearing fits anyway.

Tuff task turning a 5 mm shaft to IT6. You'd have to hit within 8 µm. Doable, but not on the first try and not on the second one.
Even if the price sticker on the lathe is ten-fold compared to a mini-lathe.

Lapping is an option, but you would have to start with something straight.


Nick
 

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