Fixing High-Pitched Whirring Electric Motor Issue

[n9643750]

My electric kitchen-aid mixer recently began to make a high pitched whirring sound that increases in pitch with the speed of the motor. After disassembly and cleaning, all of the parts seem to be damage free and operational. Any ideas what could be causing the noise?

 

[Polyamorph]

Sounds like the bearings to me...

 

[dlgoff]

What Polyamorph says. Can you grease/oil the bearings?

 

[Polyamorph]

Can you grease/oil the bearings?

If that doesn't work it shouldn't be too difficult or expensive to replace them, provided you can find a supplier that stocks identical parts.

 

[sophiecentaur]

Could it be the brushes graunching the commutator? Could you get the brushes out and have a look? But it may be an induction motor as the Kitchen Aid mixer is supposed to be high quality.
Bearings are the next most likely source of sound - or a slipping belt?

 

[n9643750]

I took apart the motor assembly as far as seemed feasible and (forgive my ignorance at what I'm looking at) found only one bearing and it didn't seem worn. It was located at one end of a shaft where the shaft fits into a socket. I'll try to describe the parts of the motor to see if it gives anyone a clue as to what sort of motor this is. The shaft has (copper?) wire wrapped around it in large bundles, and around the bundles is a set of 2 fixed magnets that the shaft spins inside of. farther towards the bearing, the wires attach to a series of metal plates that are also mounted on the shaft. the plates seem to get current through two sliding conductors (some sort of soft metal here) that are held against the shaft and energized by wound springs. Are there more bearings that I'm missing?

 

[sophiecentaur]

This sounds like a 'brush motor'.
Those "sliding conductors" are the brushes. They are normally made of carbon and make contact with the segments of the "metal plates" which are called the commutator. As the motor rotates, the brushes make contact with a different pari of plates (on opposite sides of the shaft. This ensures that the correct parts of the coil (the armature) are energised to produced the torque of the motor. If the commutator surfaces are clean, shiny copper then that's OK.
There would be a bearing at each end of the shaft but the end where the drive is taken off may be more substantial (a ball race). On the other end you may find the shaft just sits in a plane bearing (sintered phosphor bronze is common). Is this bearing free to rotate and is it clean looking?
Could the drive gear / pulley be slipping round the shaft? If it's a belt drive, is the belt shiny due, perhaps, to grease having go on it? That could be causing slippage and squealing.

 

[sophiecentaur]

Or it may be possible that the belt is slack or the motor has become mis-alligned and the belt or other drive is chafing. Look for signs of wear everywhere (everywear??).

 

[n9643750]

As this is a handheld mixer, there is no belt drive. The paddles are turned by two gears that are turned by a worm drive on the motor shaft. As for the "other bearing," I did locate a race (with no bearings inside) at the other end of the shaft (thanks!), but it also shows no signs of wear. I'm going to lubricate both ends with lithium grease (good choice?) later today and put the apparatus back together for a test.

thanks for the consideration and help.

 

[dlgoff]

As for the "other bearing," I did locate a race (with no bearings inside) at the other end of the shaft (thanks!), but it also shows no signs of wear. I'm going to lubricate both ends with lithium grease (good choice?) later today and put the apparatus back together for a test.

Lithium grease is a good choice. Let us know if that worked.

 

[n9643750]

Thanks for the help here. I've greased both bearings and am still getting the whirring noise. It's pretty hard to tell exactly where in the motor the noise is coming from. Possibly the brushes rubbing against the commutator? Can they be shaped/ replaced/ adjusted? Any other ideas would be appreciated.

 

[vk6kro]

The brushes are meant to be replaced and it is usually easy to do.

They are comparatively cheap anyway, so while you have it apart you might as well put in a new set.

Compare the old ones with the new ones to see what they should be like.
New ones have a large section of graphite attached to the metal while old ones can be mostly metal.

If they are very worn, have a look at the commutator to see if it has been damaged.

 

[sophiecentaur]

Those plain bearings can cause trouble in automotive motors and alternators. They are made of 'sintered' metal, which is a spongy-like aggregate made up of lots of grains of metal (bronze?) which are pressed together hard. The spaces in between are suppose to conduct the lubricant onto the bearing surface. If ever they dry out, the wear and heat tends to burnish over the holes in the bearing surface and no more lubricant gets through. It's a runaway situation. In some cases, it is possible to replace squealing bearings but, in this case, I doubt it.
You may need to do your mixing with earmuffs!
Good luck and try the brushes solution.