Makita GB800 bench grinder very hot to touch -- is this "normal"?

**Plectropomus** · 4th May 2021, 11:48 AM

Hi brains trust,
I have an issue with heating of the motor case on my bench grinder.

I inherited a Makita GB800 8inch bench grinder that had never been used. For a variety of reasons, it stayed unused for 20 years, in storage inside the box in a variety of humid and hot conditions in the tropics.

Now I finally have a workshop I have begun using it and noticed the casing was very hot to the touch.

Tonight I ran it for (timed) 20 minutes with no workload at all, and at the end of this time I could leave my right index finger on it for 6 seconds. I could leave my left index finger on it for just 2 seconds. I am right handed, so perhaps my right finger is "work hardened"

The heat is definitely emanating from the centre of the machine -- not the end bearings. The wheels spin freely and there is no noise of any remark. There is some vibration, and it may be because I have not "trued" the wheels. I believe both wheels came with the machine.

There are holes under the casing for aeration of the motor cavity, and plastic fans are visible inside. I am concerned that humidity during storage may have affected the electrical components somehow.

My questions are: (1) Should I be concerned, (2) is this "normal"?, and (3) what could I test using a multimeter for faults?

The Googling I have done provides contradictory information -- some say a 20 minute run under no load should be "cool to the touch", and others say it could be quite hot to touch, as the motors are designed to operate at high temperatures. I have no infrared temperature gauge and live in a rural area, so I have no accurate information on the ambient temperature or the temperature of the casing.

Can you advise?

Cheers, Mike

**BobL** · 4th May 2021, 12:48 PM

The general cooling for single phase bench grinder motors compared to stand alone single phase electric motors is minimal because
a) they are not very efficient - ie instead of covering electrical energy to rotational energy they do generate heat.
and
b) they are not expected to run for more than a few minutes at a time.
So 20 minutes free running and getting hot is fairly normal.

Ive had several budget end grinders that would get very hot running for 20 minutes, but not quite to the 2 second touch point you mention.

Sometimes the overheating can be due to capacitor problems eg going out of spec, so you could try replacing it/them

**Plectropomus** · 4th May 2021, 01:27 PM

Originally Posted by BobL

The general cooling for single phase bench grinder motors compared to stand alone single phase electric motors is minimal because
a) they are not very efficient - ie instead of covering electrical energy to rotational energy they do generate heat.
and
b) they are not expected to run for more than a few minutes at a time.
So 20 minutes free running and getting hot is fairly normal.

Ive had several budget end grinders that would get very hot running for 20 minutes, but not quite to the 2 second touch point you mention.

Sometimes the overheating can be due to capacitor problems eg going out of spec, so you could try replacing it/them

Thanks, BobL,

That gives me both reassurance and a place to start troubleshooting.

I am new to the bench-grinder, but have watched YouTube videos of fellows hogging off huge amounts of material to make knives from old files (etc), so I presumed long running, with reasonable loads, was the norm. Perhaps they are using "industrial" grade bench grinders?
Cheers, Mike

**BobL** · 4th May 2021, 01:48 PM

Originally Posted by Plectropomus

Thanks, BobL,

That gives me both reassurance and a place to start troubleshooting.

I am new to the bench-grinder, but have watched YouTube videos of fellows hogging off huge amounts of material to make knives from old files (etc), so I presumed long running, with reasonable loads, was the norm. Perhaps they are using "industrial" grade bench grinders?
Cheers, Mike

Probably using three phase grinders which are more efficient and smoother,and can run all day.
Serious knife makes will use linishers (belt sanders) with a variable speed, often powered by a 3 phase motor.

AND to more specifically illustrate this, I just happened to be using a 3HP grinder for about an hour and the warmest it got was 36º

**Colin62** · 4th May 2021, 05:47 PM

If it’s not the bearings, I’d be looking at the capacitor next, they don’t last forever.

**Greg Q** · 9th May 2021, 08:31 PM

Could critters have taken up residence, blocking whatever cooling air from free flow? I spend ten minutes in the tropics and get bugs...20 years would see me plugged right up.

**BobL** · 9th May 2021, 10:03 PM

Originally Posted by Greg Q

Could critters have taken up residence, blocking whatever cooling air from free flow? I spend ten minutes in the tropics and get bugs...20 years would see me plugged right up.

This happened to a mens shed drill press motor when wasps made a nest in it.
It fair cooked the motor.
Wasp1.jpg Wasp2.jpg

**LanceC** · 10th May 2021, 12:31 PM

Originally Posted by Colin62

If it’s not the bearings, I’d be looking at the capacitor next, they don’t last forever.

And Bob's very informative post about VFD storage introduced many of us the the fact that electrolytic capacitors degrade in storage, so a brand new capacitor that's been in a box for 20 years (as in this case) will certainly be in less than new condition.

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