Woodcraft Slow Speed Grinder UPDATE - Hot is OK

[Doug Olsen]

Woodcraft Slow Speed Grinder UPDATE - Hot is NOT OK

HOT is NOT ok. I've returned the grinder to my local Woodcraft. The owner and a couple employees all agree that the grinder should not run. Owner will contact Woodcraft HQ about this issue. Looks like a bad batch of grinders.

This is an update to the earlier thread regarding Woodcraft's slow speed grinder getting very hot.

Today, I took my hot running grinder to Woodcraft for an exchange. There were two more at the store, both of which were tested and these ran hot after about 15 minutes. So that is three!

The gentleman at the store called Woodcraft HQ technical department and discussed the hot running grinders. Tech support acknowledged receiving complaints form other Woodcraft stores so they took a dozen grinders out of the warehouse and ran them for a protracted period of time. In every case they ultimately got very hot and their conclusion is that this is normal.

It appears this is inherent in the current crop of grinders offered by Woodcraft. Ergo, hot running is apparently OK and unless it burns up, I'll keep it .

I took the grinder home and returned it to operation. Doug

 

[Bill Boehme]

Well, they might be correct, but their conclusion was not based on a good investigation. More correctly, they should have compared the performance against an older motor and then tried to determine whether a different type of motor was being used if there really was a difference in temperature rise. From what they did, another conclusion could have been that something in manufacturing changed and they were putting out a batch of defective motors.

The nameplate should indicate the NEMA temperature class of the motor and you can find out more about that HERE. Based on the temperature rating, the temperature in the column labeled "Allowable Temperature Rise at full load" is what you should go by. Each 10° C rise above that number cuts motor life by 50%.

It could be that they switched to a PSC (permanent split capacitor) motor which tends to run hotter than capacitor start motors. If the manufacturer forgot to do something like put oil in the packing for the bushings (assuming that the motor uses bushings rather than bearings) or that spacers between the rotor and frame create a too tight fit (you could tell if that is a problem by checking to ensure that the shaft spins freely). If the fit is tight then it probably will "wear in" rather than "wear out".

 

[odie]

Question: Wouldn't a bushing type motor require some regular method of maintenance lubrication.....oil, or grease?

Permanently sealed bearings seems like the way to go these days. I know of some oil impregnated bronze bushings on some of the equipment where I work, but these are relatively low rpm bushings......less than 100rpm, I'd guess. These do not require regular application of lubrication. Something like this would probably not work for a higher speed application......such as a slow speed grinder.

ooc

 

[john lucas]

None of the Woodcraft grinders I've used had bushing that needed oiling.

 

[Bill Boehme]

Question: Wouldn't a bushing type motor require some regular method of maintenance lubrication.....oil, or grease?

Permanently sealed bearings seems like the way to go these days. I know of some oil impregnated bronze bushings on some of the equipment where I work, but these are relatively low rpm bushings......less than 100rpm, I'd guess. These do not require regular application of lubrication. Something like this would probably not work for a higher speed application......such as a slow speed grinder.

ooc

Most PSC motors that are less than about 1/2 HP or so use oiled bushings and run at speeds from roughly 800 to 1700 RPM. Their main use is probably in HVAC systems or anything that doesn't require much starting torque. In the past they had to be oiled periodically, but many of the newer ones do not have provisions for oiling them. The way that they work is that the bushing is surrounded by a wadding material that is saturated in oil and there is a hole in the bushing with a wick that keeps the space between the shaft and bushing lubricated. There is a small disk at each side of the bushing called a "flinger" because it flings the oil back into the chamber with the oil soaked wadding. Therefore, this constitutes a closed loop oiling system that supposedly needs no maintenance. In the real world, that theory is a bit optimistic.

I suspect that many low cost bench grinders use PSC motors because they are much cheaper to make and the duty cycle for grinders is much less than it is for an air handler. It is common for this type of motor to be either TEAO (totally enclosed air over) or ODPAO (open drip proof air over). In an air handler application this is not a problem to provide the cooling air, but I have never seen a bench grinder with a fan. If the motor is operated for only a few minutes and allowed to recover for a much longer period then operating without a fan is not a problem.

I just recently rebuilt a PSC TEAO motor and have been bench testing it without a fan to see what the temperature rise is. My measuring instrunent was my hand -- if I can keep my hand on it then I am happy. Initially, things were rather tight so it did not take long to get blazing hot. After about a dozen sessions of that, things loosened up to where the shaft turns freely and I can run it as long as I want and am still able to keep my hand on it although it would be better to have a fan cooling it down a bit.

 

[odie]

Most PSC motors that are less than about 1/2 HP or so use oiled bushings and run at speeds from roughly 800 to 1700 RPM. Their main use is probably in HVAC systems or anything that doesn't require much starting torque. In the past they had to be oiled periodically, but many of the newer ones do not have provisions for oiling them. The way that they work is that the bushing is surrounded by a wadding material that is saturated in oil and there is a hole in the bushing with a wick that keeps the space between the shaft and bushing lubricated. There is a small disk at each side of the bushing called a "flinger" because it flings the oil back into the chamber with the oil soaked wadding. Therefore, this constitutes a closed loop oiling system that supposedly needs no maintenance. In the real world, that theory is a bit optimistic.

I suspect that many low cost bench grinders use PSC motors because they are much cheaper to make and the duty cycle for grinders is much less than it is for an air handler. It is common for this type of motor to be either TEAO (totally enclosed air over) or ODPAO (open drip proof air over). In an air handler application this is not a problem to provide the cooling air, but I have never seen a bench grinder with a fan. If the motor is operated for only a few minutes and allowed to recover for a much longer period then operating without a fan is not a problem.

I just recently rebuilt a PSC TEAO motor and have been bench testing it without a fan to see what the temperature rise is. My measuring instrunent was my hand -- if I can keep my hand on it then I am happy. Initially, things were rather tight so it did not take long to get blazing hot. After about a dozen sessions of that, things loosened up to where the shaft turns freely and I can run it as long as I want and am still able to keep my hand on it although it would be better to have a fan cooling it down a bit.

OK, thanks Bill......

That does seem to make sense.

I'm wondering......do you know what kind of spindle bearings are currently being used for the VB36 lathe? I know it's a bushing arrangement, but I'm wondering if it's the enclosed wick and oil wad arrangement you mention here......or, something that requires manual lubrication........

ooc

 

[Bill Boehme]

OK, thanks Bill......

That does seem to make sense.

I'm wondering......do you know what kind of spindle bearings are currently being used for the VB36 lathe? I know it's a bushing arrangement, but I'm wondering if it's the enclosed wick and oil wad arrangement you mention here......or, something that requires manual lubrication........

ooc

I read something about it once and seem to remember that it is in some sort of oil bath arrangement. I believe that it has fill and drain plugs and might even have a sight glass or dipstick.

 

[Ian Robertson]

OK, thanks Bill......

That does seem to make sense.

I'm wondering......do you know what kind of spindle bearings are currently being used for the VB36 lathe? I know it's a bushing arrangement, but I'm wondering if it's the enclosed wick and oil wad arrangement you mention here......or, something that requires manual lubrication........

ooc

The VB56 uses a bush system with oilers on top. This is taken straight from the brochure available here http://www.hegner.co.uk/catalog/partdetail.aspx?partno=VB36:

NO NOISE, NO WEAR, NO “FLOATâ€
Another limitation imposed by cost considerations in the above case
(and with most other lathe designs) is seen in the choice of bearings.
Although transmuted into a virtue by marketing men, ball bearings
are not the ideal support for heavily loaded shafts running at low
speeds. Radial ball bearing assemblies and, to a lesser extent, roller
bearings, are specified for reasons of economy as they are relatively
cheap to buy and demand far less rigid engineering tolerances in the
machining of the housings in which they are fitted than precision
plain bearings. However, where extreme load carrying capability
is called for, especially at low rotational speeds, plain bearings are
the natural choice because they effectively eliminate metal-to-metal
contact at the bearing interface. Such bearings often have the capacity
to carry loads far in excess of the actual machine frame strength.
(The Rolls Royce RB211 turbine developing 38,000 shaft horse
power has plain mainshaft bearings lubricated by compressed air.
Industrial machines from watchmaker’s lathes to micron accurate
CNC grinders also use plain bearings.)
Consider the total silence with which these bearings do their job; the
potentially perfect rotational concentricity they provide (effectively
zero shaft float); mind-boggling load carrying capacity, and a life
expectancy far exceeding one’s own, and you can understand why
this is an essential requirement for the “perfect†lathe.

 

[Dennis J Gooding]

Doug, I'll take the contrarian point of view. If that much heat is being developed by friction of the bearings, I will bet on the bearings failing very soon now. It seems more likely to me that the heat is coming from resistive and eddy current losses in the motor itself caused by inadequate wire size or other design factors. The fact that other motors like it also run hot should not be surprising.

Dennis

 

[Jerry Bennett]

Doug, just because other units in the store get hot does not mean it is OK. Just because the people in the store thinks it is OK, does not mean it is. Heat is a result of energy being expended. If there is no load and it is getting hot, there is a reason. What is causing an internal load that would expend that much energy? A correctly designed motor will heat up based upon the external load applied. With no load, there should be little. Excessive heat will degrade the windings and could in time cause a fire.

I would take it back and get another brand.

Jerry

 

[Bill Boehme]

Something else to consider -- there are many different types of electric motors intended for optimal performance for a wide variety of uses. Suppose for example that the motor being used is a permanent split capacitor type normally used as an air handler in HVAC systems instead of the common everyday capacitor start induction motor. A PSC motor has very low starting torque which is just fine for a bench grinder, but air handler motors are designed for a constant full speed load such as what would exist with a squirrel cage blower. That is where they reach their peak efficiency. If they are operated unloaded which is essentially the case with a grinder, the efficiency is very low which means that a lot of energy is being wasted in doing nothing but heating the motor. So loafing on the job doesn't necessarily mean that you won't work up a sweat if you work hard at loafing.

 

[Jerry Bennett]

This is true Bill regarding motors for HVAC and dust collection systems. Generally there are warnings on such motors not to operate without a load. The windings are designed such that the current and voltage peaks are at optimum locations under a specific load. Under no load conditions, they lag one another causing an unwanted internal current load. A motor such as this should never be found on a grinder.

Unusual heating in a grinder motor is a design flaw or mfg flaw. Really not a good idea to put into service.

Jer

 

[robo hippy]

If I had a grinder that got that hot, I would demand a refund. They are not supposed to get that way, the people in the store should know that, and if the others they tested turn out that way, then they got a whole batch of bad grinders. Time to call the manufacturer.

robo hippy

 

[Doug Olsen]

Hot is not OK

I returned the Woodcraft Grinder for a refund. Employees and the owner of this store all believe that there is a problem with the latest batch of grinders.
They should not run hot.
Regards,