Treadmill motors - my modifications.

Treadmill permanent magnet DC motors are popular amongst home machinists because they are readily available from scrapyards and similar, as well as being easy for variable speed.
This post shows how I modify such motors to make them more suitable for the task of driving machinery such as small lathes, drills, mills and grinders. Some unusual machining techniques are demonstrated. The main focus is on grinders because they have addition problems due to magnetic grinding dust.

The video shows several areas of attention which needed improvement such as bearing fit, bearing alignment, general rigidity and for grinding use the need for sealing. Some unusual machining techniques are demonstrated.

https://www.youtube.com/watch?v=wdvPHtyhYsc

Attachment 38011 The objects in question. (click images for full size.)

Attachment 38005 Attachment 38006

Attachment 38007 Attachment 38008

Before and after main bearing carrier (above).

Attachment 38009 Attachment 38010

Truing the casing.

Attachment 38012 Truing the flat belt pulley on the armature.

Attachment 38013 All the bits.

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Here is one of the motors mounted on the grinding head for my T&C grinder. See https://www.homemadetools.net/forum/...6205#post83247

Attachment 38066 Attachment 38067 Click images for full size.

1. I like this modification but have concerns over the heat generated, particularly if used for a belt grinder. A normal fan ventilated motor gets some respite from full load heat when the load is removed and the motor is still running and ventilating. A fully enclosed motor is subject to a continuous build up of heat, slightly less when off load, but still there, and it has nowhere to go. Sealed motors usually have fins on the outside and if necessary have a fan attached to cool those fins.

2. Fitting mesh filters at each end will do very little to improve cooling unless you retain the fan to draw the air through the armature area.

3. Lowering the speed by rheostat reduces the torque as you pointed out but I understand that there may be better electronic speed controllers available that allow a full range of speeds without torque loss. Those fitted on the more expensive treadmills tend to have no loss of torque with speed reduction.

4. Retaining the original flywheel fan for its flywheel effect and for the cooling air it might draw over the outside of the motor might help with the cooling. Heat sinks attached to the motor body and retaining the flywheel fan would help more. Many people utilise the treadmill speed controller and RPM indicators. The speed indicators are driven by a sensor on the flywheel, another good reason for keeping it. It is common to attach the drive pulley directly to the flywheel by bolting or welding and your way of truing it up would be perfect for this as well.

5. There is much debate on the web about whether the drive wheel should be crowned on a belt grinder. Suppliers of ready made wheels generally supply crowned drive and tracking wheels but that doesn’t make it right. There is an excellent explanation somewhere on the web, (I had thought it was your excellent website tonyfoale), that explains how only the tracking wheel should be crowned. If both wheels are crowned they tend to fight each other for alignment.

6. I wrote this assuming that you were building a belt grinder but now see pics of a tool & cutter grinder. Heat is going to be less of a problem with such a grinder and the relatively light loads involved. May it last you forever.

Quote:

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Originally Posted by Moby Duck

1. I like this modification but have concerns over the heat generated, particularly if used for a belt grinder. A normal fan ventilated motor gets some respite from full load heat when the load is removed and the motor is still running and ventilating. A fully enclosed motor is subject to a continuous build up of heat, slightly less when off load, but still there, and it has nowhere to go. Sealed motors usually have fins on the outside and if necessary have a fan attached to cool those fins.

2. Fitting mesh filters at each end will do very little to improve cooling unless you retain the fan to draw the air through the armature area.

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Watch the end part of the video. The heat considerations are explained there. See response to your point 4.

Quote:

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Originally Posted by Moby Duck

3. Lowering the speed by rheostat reduces the torque as you pointed out but I understand that there may be better electronic speed controllers available that allow a full range of speeds without torque loss. Those fitted on the more expensive treadmills tend to have no loss of torque with speed reduction.

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I did not mention rheostat. It never occurred to me that anyone would try to control the speed in that way. A rheostat is more like a torque control than it is a speed control. Totally unsuitable.

Quote:

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Originally Posted by Moby Duck

4. Retaining the original flywheel fan for its flywheel effect and for the cooling air it might draw over the outside of the motor might help with the cooling. Heat sinks attached to the motor body and retaining the flywheel fan would help more.

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These are rotating armature motors and so most of the losses will be in the rotating bits and the cooling heat path will be out through the bearings and the pulley. Cooling air over the outside body will have only a small effect, on the other hand cooling air on the inside, as per a stock motor is effective. On the other hand the common squirrel cage induction motors have a fixed armature attached to the casing and so air drawn over the outside of the casing will cool. That is why those motors often have external fins with a fan designed to flow air over the fins and not through the inside. All motors are not equal.

Quote:

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Originally Posted by Moby Duck

Many people utilise the treadmill speed controller and RPM indicators. The speed indicators are driven by a sensor on the flywheel, another good reason for keeping it. It is common to attach the drive pulley directly to the flywheel by bolting or welding and your way of truing it up would be perfect for this as well.

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You can just as easily put the sensor magnet on any pulley. If the treadmill motor controller is available then that is certainly the easiest way to get a speed control. I do not use one because they have built in delays and other safety features which make the control feel unresponsive. I use the PWM feature of an Arduino to drive a couple of IGBTs. Instant response.

Quote:

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Originally Posted by Moby Duck

5. There is much debate on the web about whether the drive wheel should be crowned on a belt grinder. Suppliers of ready made wheels generally supply crowned drive and tracking wheels but that doesn’t make it right. There is an excellent explanation somewhere on the web, (I had thought it was your excellent website tonyfoale), that explains how only the tracking wheel should be crowned. If both wheels are crowned they tend to fight each other for alignment.

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I don't know why there is any debate on this. Flat belts have been used for hundreds of years to drive machinery and the correct methods are well known. I thought the explanation that you mention was on this forum but a search did not throw it up so I have made a new post https://www.homemadetools.net/forum/...521#post173916

Quote:

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Originally Posted by Moby Duck

6. I wrote this assuming that you were building a belt grinder but now see pics of a tool & cutter grinder. Heat is going to be less of a problem with such a grinder and the relatively light loads involved. May it last you forever.

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Watch the end part of the video. I do not think that I would use one of these motors on a belt grinder. Semi-continuous high loads need cooling air through but the air will contain a lot of dust, even with a dust control vac. A T&C grinder generates very different conditions.

Tony, this motor may not have issues with removing the rotor from the stator housing with the permanent magnets, but there are PM motors that require "keepers" when the rotor is removed to keep the magnetic path in place. They will loose considerable magnetism if this is not done. Stepper motors have this warning. I believe it is an issue with extremely high power rare earth magnets used in high torque miniaturized motors. I think this notice should be known.

The main problems which cause loss of magnetism in modern magnets is heat and strong demagnetising fields.

In motors they are subject to demagnetising forces due to the back EMF produced by the motor rotation. Of course these must be way below the threshold or motors would quickly cease to work.

"Pure" rare earth magnet material has a low Curie temperature, that is the temperature above which they lose their properties. In order to make practical magnets for use at the temperatures in motors etc. The material has to be mixed with other stuff.

The main problem with pulling armatures from such motors is trapping your fingers when reassembling. Careless handling of modern magnets is a cause of many finger injuries.

The N42 magnets a customer uses in some products I designed for him are only good to 80C. Pretty slim margin in some of his uses, but it's what it is.

Quote:

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Originally Posted by jdurand

The N42 magnets a customer uses in some products I designed for him are only good to 80C. Pretty slim margin in some of his uses, but it's what it is.

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Yep, a slim margin. The rare earth magnets have to compromise between strength and operating temperature, not forgetting cost.

I should also mention that the high power magnets are under a LOT of internal stress and can shatter with quite a force. I machined up a metric krap load of magnet brackets for another customer* who's been doing some sort of power experiments. He reported that something came loose and the resulting multiple magnet crash shattered some of them and pieces dented cabinets and such but all missed him. He's decided to be a bit more careful with these things.


* last paid project before selling off our machine shop equipment.