Motor 'pulsing'

[Martyn K]

Hi all

Last weekend the 4mm propshaft of my 3536 motor sheared off midflight. The motor was bought off fleabay along with a 60A ESC and in its very short life the matched pair ran perfectly.

It was this package

To replace the motor, I ordered a similar spec (1700kV 3536) from Gaint Shark expecting (hoping) it should be a simple exchange.

The motor duly arrived in a very timely manner (thanks GC) and was fitted last night. Even the mounting holes lined up. So far so good.

However, with the same ESC and the same prop (8x6e), the motor pulses when on load. By pulsing, I mean that it is peaking at about 7000 rpm on a 3S lipo but the revs are rising & falling by about 2000rpm in about a 2(ish) second repetition frequency. Off load or with a smaller prop, it picks up and revs as expected.

The previous motor had no problems running an 8x6e.

What have I done wrong?

Thanks

Martyn

[Chris Bott - Moderator]

Could it be the Lov Voltage Cutoff in the ESC Martyn? Sometimes they can be set to gently slow the motor rather than shut it off completely. I'm thinking that as it starts to slow the motor, the battery voltage recovers so it takes away the cutoff again... ad infinitum.

[Martyn K]

Thanks Chris

I have tried two different LiPo's. I could eliminate that by adding a seperate receiver battery (I think). I'll try later

Best wishes

Martyn

[Chris Bott - Moderator]

I'm not sure that would help Martyn. If it does happen to be the LVC, may still slow the motor as volts drop.

Can you program the LVC to do something different, then see if theres a different symptom?
Checking the LiPo voltage while at full rev smight reveal something too.

[Olly P]

Martyn - also check the settings on the ESC- the cut-off may be set too high.

[John Cole]

I've looked at the package details and they recommend a 8*4 not 8*6. Maybe you are just over-propping the motor and thereby overloading the ESC. Have you access to a wattmemeter to check the current to the ESC?

[Martyn K]

Hi all

Some progress.On the grounds that I couldn't find the programming info, I took the easy option and went out and bought a couple of APC 8x4e props. That appeared to cure the problem - no pulsing.

Many thanks.

So, with fuselage in one hand I opened it up with the idea of measuring peak current/rpm.

Then the prop shaft snapped - again. (A 4mm shaft)

This is a brand new motor, and had run for probably less than a minute.

The prop is balanced, there were no vibrations, the shaft simply snapped at the circlip, just like the previous one...

(sorry for the repeat photo - I have a matching one that I have not uploaded yet but with a red motor)

*Please* - just what am I doing wrong here?. I cant believe that I have 2 matching faulty shafts, so I assume that the fault is mine.

The measurements are approx, but there is about 8mm between the motor mount and the the rear of the prop driver/spinner assembly.

Any suggestions? This is getting silly/expensive/frustrating/etc..

best wishes

Martyn

 

 

Edited By Martyn K on 25/10/2012 22:48:21

[Ian Jones]

Take heart lad, we'll think of something... think.... think... think...

...ooh nothing happened .

You could always put an AP15 in it.... oops sorry.

Dropped you a PM.

Ian

[John Cole]

I take it hat this is a pic of the original motor with the original prop adapter. Did you use the old prop adapter on the new motor? If so then it could it be that the prop adapter is faulty - out of centre? That would put the prop assembly badly out of balance and put a big sideways load on the shaft, as it is quite long.

You don't say which new motor you bought. As it's red I guess it's either a N3530/07-KV1700 or a N3536/04-KV1700, and probably the latter as your previous motor was sized 3536.. I note that there is no customer feedback on either of these motors.

You might try a well-liked motor such as the E-Power GC GT2812 1840Kv with a 5 mm shaft (and its own prop adapter). Or an XYH35-36 1450. GS list spare shats for the latter, but none in stock!

[Martyn K]

Hi John

I used the prop adaptor from the original. It didn't appear to be out of alignment, in fact one of the checks that I perform is that it is spinning true before I really open it up. If it was out of alignment, I would have expected that it would be bibrating like mad, but it was running very smoothly, even at full bore.

It was the N3536/04-KV1700 that I bought. I was going to order a spare shaft but as you spotted they are nil stock. However, AirTek had them in stock and 2 shafts arrived this morning, but without the circlip groove. I am not worried about that as the motor is pulling not pushing so it is not really required.

Interestingly, the GS motor arrived with an alternative prop adaptor that can be bolted to the rotor, however, I couldn't get that to fit in the existing layout area. I wont be able to use it with the replacement propshaft anyway as the circlip has now gone, (it will effectively be pulling the motor apart - the motor will have to be mounted backwards.

One thing that I may try is to put a PTFE support bearing in front of the motor in the engine bulkhead to give the shaft a bit more support.

All very frustrating...

Martyn

[Myron Beaumont]

Ian Jones

You took the words out of my mouth about using an AP .15 . Cracking little motor and I've never had a shearing off crankshaft /propshaft problem .

edit -- Whoops ,I said motor .I meant "engine"

Edited By Myron Beaumont on 26/10/2012 12:07:24

[Martyn K]

Hi Myron

I have to admit that I am a little peeved with this. It is meant to be my winter hack when I cant/dont want to lug fuel starter etc to the field.

I have never seen anything like this. Can you imagine how difficult it is to fracture 4mm diameter hardened steel?

Martyn

[John Cole]

Did the GS shaft fracture at the circlip groove? If so then it's interesting that the replacements lack that groove. Likely that a shaft will fracture at a crack (hardening makes that even more likely). As the motor was new any such crack must be formed in manufacture, and a badly-cut groove is a possible source.

Good luck with the new shaft!

[Myron Beaumont]

Martyn

I too have a few lekky powered models for more or less the same reasons .My problem has been the bending of those tiny dia. shafts . At least with a bent shaft I can straighten it .I think maybe the far eastern guys have a little to learn about metallurgy .All that scrap metal they import that we & others sell them needs to be re constituted after melting it all together to get to the right metal for the right job .I hope they never start making gas turbine engines ! (Ex-RR engineer)

[Myron Beaumont]

John

X posted you .Yes.I wonder just what the effective dia. is at the circlip groove .I think I see the circlip on the photo .

Edited By Myron Beaumont on 26/10/2012 12:47:17

[Martyn K]

well....

 

John - yes both failed in the same place in the circlip groove. The first failure had a highly granular structure to the fracture, the second was finer grained.

I have just replaced the shaft on the red motor (GS) with an airtek replacement, (I had to grind a set screw flat. It was tricky to extract -- needed a bit of heat, but threplacement went back in OK.

Reassembled the whole lot and soak tested it by running if flat out for about 20 seconds and it didn't break. I hope to give it a more exhaustive test (at the field) this afternoon if I can escape early enough.

Thanks all for help/encouragement/ideas. Fingers crossed it lasts longer than a flight..

Best wishes all.

 

Martyn

Edited By Martyn K on 26/10/2012 13:51:46

[Dave Bran]

Looking at the original pic in the Ad, I'll offer a possibly less obvious but still likely cause.

I have had a couple of those ESCs, they are sold with various labels, just like most Chinese stuff, but the givaway is the colouration used, shape, the heatsink type and the way that the sleeve is cut to give a heat outlet only over part of the heatsink surface. Basically, they are rubbish!! I had one that kept resetting itself, another just didn't throttle smoothly, both times fitting a still cheap Hobbywing replacement stopped the symptoms dead. No way will I buy any more.

To get pulsing as you describe it can only really be a ESC issue, the fact that you didn't notice any the first time around or after using a lower pitch prop does not mean the ESC wasn't high frequency pulsing, putting abnormal backlash twist loading on what will always be the likely failure point, the sharp edged circlip groove.

Incidentaly, having been heavily into EP since 2006, and with currently over 40 EP models of all sizes from tiny floaters to outright screamers, I have only had one shaft failure on a belly landing prop strike and not a single one bend, that is if you discount the completely hopeless so called motors used in the Flying Leg-Ends carp!!!

Ok, a less brittle shaft may stand it, but if you had a properly balanced prop and prop holder, and experienced what you did, then from my experience of those ESCs, that is your issue.

I have LOTS of HobbyWing ESCS, only one has failed, on a 500 Heli, and that was helped along a fair bit by me. (OOPS) Not one has needed tweaking for any motor other than to add a brake where appropriate.

Good Luck!

[Martyn K]

Thanks Dave

That looks like sound advice and experience. I'll try a different ESC.

Best wishes

Martyn