Elec Motor Brake Help

[JohnW]

I have a Dewalt RAS GW model, mid 1950's with the original motor. It's a great saw in excellent condition and works very well. But when switched off the blade spins for 3 1/2 minuets which I see as a safety issue. I'm not even sure if this saw had any kind of a mechanical or electrical brake on it when new.

Anyone have any knowledge about this? Does it have a brake, and if so, what should I check or do to fix it.

 

[Mark Gottesman]

Someone here may know, but other sources of info would be the Dewalt RAS forum on Delphi and the Old Woodworking Machine forum (OWWM..org).

First thought I have is the bearing grease has broken down and the machine is freewheeling. Some RAS had a manual brake that could be added to the outboard shaft end. Worked sort of like a coaster brake.

 

[mkepke]

I think those are both good suggestions (although my gut says the GW did not come from the factory with a brake - I owned the larger GA-type and it didn't come with one).

I believe you could get an aftermarket electronic brake at one time. Or make your own:
http://www.sawmillcreek.org/showthread.php?177537-Anyone-build-an-electric-brake-for-their-RAS

-Mark

 

[Bill Clemmons]

I have a 12" DeWalt RAS from the late 80's, and it does not have a break. It also spins for quite awhile after shutting it off.

 

[chris_goris]

I have an old State 20" disc sander that free wheels for about 6-7 minutes after power off, I attibute it to good bearings. If I want to shut it down sooner, I just keep an old scrap nearby and grind it off till it stops, I guess you could do the same with a RAS.

 

[ehpoole]

I do not know what type motor your RAS uses, but there are two types of electrodynamic braking possible depending upon motor type.

If it is permanent magnet motor then it will generate power when unpowered and you can short the lines (experiment with low value power resistors first as a dead short may result in too much braking torque). You may be able to do the same for a universal motor but it would require that the field retain sufficient magnetism to generate power, otherwise power will have to be provided to the field coils (to magnetize them) to allow the motor to generate power so that it can be stopped.

If it is an induction motor then it can be braked by passing a current limited low voltage DC current into the unpowered motor. The rotor will then attempt to align with the now stationary magnetic fields in the stator, creating a braking force within the motor. Start off with a modest voltage and current to begin with and stop the current as soon as the blade comes to a stop. If you are too aggressive or leave the current flowing uninterrupted you can either overheat the stator windings or, if far too much current is used, you can create such a strong braking force that the motor or equipment is damaged from the sudden braking force.

You would have to experiment with the above to find out what works for your setup, but only attempt such if you are competent working with electrical as you will be tinkering with exposed line voltage (120/240) while experimenting. You must also ensure that things are properly interlocked so as to ensure that the braking system can never be activated while the motor is energized.

You could also experiment with Lorenz braking using an electromagnet with the two ends of the magnet wrapping around to each side of the blade. However, a typical saw blade may be too thin to accept the heat from the induced eddy currents without overheating.

Or you can simply keep a block of hard scrap lumber around to make one last "cut" after powering off to stop the blade, just do so safely.

 

[JohnW]

As suggested, I went to the OWWM site and it's sister sites and found some interesting info.

1st - I'm not comfortable enough with motors to experiment with installing a DC braking system. There are some great links for doing this on these sites...but I'm not going down that road.

Here is what really has me scratching my chin/head. As Chris says above, his old 20" disc sander spins for a long time after shut-off and he attributes that to good bearings. I was thinking the same thing about my RAS. But there are lots of references on OWWM suggesting the long spin time is an indication that you need to change bearings???

Found one article showing how to change the bearing on a motor similar (not exact) to mine. The bearing has a disc next to it and it was labeled "brake". The instructions say to rough this disc up a little (take off the shine) and combined with a new bearing, the brake would work better and the spinning is reduced.

Finally, I saw where someone made a mechanical lever with a hunk of leather on the end. He uses that to rub on the blade and stop it. Now that I can do....

 

[mkepke]

<snip>
Here is what really has me scratching my chin/head. As Chris says above, his old 20" disc sander spins for a long time after shut-off and he attributes that to good bearings. I was thinking the same thing about my RAS. But there are lots of references on OWWM suggesting the long spin time is an indication that you need to change bearings???

<snip>

Seems counter intuitive, doesn't it ?

Here's a simple experiment that should shed some light: If you happen to have purchased a machine with a pretty new induction motor recently (or know a friend that has), unhook the belt (if it has one) and turn the motor on. Then turn it off and measure how long the new motor - with its factory-spec new bearings - spins.

Chris' big ole State sander has a big flywheel, the sanding disc, which will tend to keep it spinning. You see the flywheel effect for yourself if for example your test motor has a big pulley on it.

-Mark

 

[JohnW]

Thanks Mark...but I get the flywheel effect....just not understanding why excessively long spinning is an indication of worn bearings. Seems to me a bad bearing would cause the spinning to stop faster due to increased friction.

Here's a pic of the RAS. Missing the anti kick back pawl but it stays in the 90 degree position 100% of the time.

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[ehpoole]

Thanks Mark...but I get the flywheel effect....just not understanding why excessively long spinning is an indication of worn bearings. Seems to me a bad bearing would cause the spinning to stop faster due to increased friction.

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It helps to realize that grease is very viscous and thus has a slowing effect on the rotation. When the grease dries up and disappears that viscosity is lost and it is just polished metal on polished metal, very little friction when there are no lateral forces being applied to the bearing as centrifugal force makes the system self centering with little load. As the bearing eventually wears more due to the loss of lubrication then things will begin to behave more as you might expect, especially with lateral loading applied.

 

[mkepke]

Thanks Mark...but I get the flywheel effect....just not understanding why excessively long spinning is an indication of worn bearings. Seems to me a bad bearing would cause the spinning to stop faster due to increased friction.

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The CW is that excessively long spin down times indicate that the seals may be excessively worn (this would be especially true in a sealed bearing since seals in good condition generate considerable friction) and/or the grease is no longer functioning properly. Grease is supposed to be sticky and remain spread through the race to provide continuous lubrication (and causing some friction).

Absent other symptoms I don't consider this long running behaviour a cause for immediate panic (unlike running a car engine w/o oil) rather a warning that the bearings are reaching the end of their maintenance-free life. That means maintenance or replacement sooner rather than later.

My suggestion to look at the spin down times of a fairly new motor was so you could calibrate your expectations: is 3 minutes long or short ? Is 6 minutes long or short ? A fairly new motor in a hobby shop should have bearings in near optimal condition: greased and installed to factory spec, broken in, but not used up or contaminated.

-Mark