Thread: Reversing the direction of my Belt Sander?...

Most single direction armatures are wound in such a way that if you did reverse the stator you would get very little tourque.

Machines like sanders are single direction and the lead in the windings gives more torque in one direction.

I would say it would not be satisfactory and in such a cheap machine a waste of time.

There are a couple of things involved here.

1. To reverse the motor rotation, you need to reverse the mag fields greated by the armature windings relative to those produced by the stator windings. As you surmised this is done by reversing the wires at the brush holders. So why does it run rough and arc a lot when you do this?
2. The armature is 'timed' i.e. the armature windings are advanced part of a segment relative to the comutator segments to maximise torque and minimise arcing. When you reverse the direction of rotation, this timing is effectively retarded instead of advanced reducing torque and increasing arcing.

To reverse the motor efficiently, a different armature with the timing reversed would be required. To make it switchable F/R, then a neutral timed armature can be used, but at the expense of an increase in arcing and a reduction in torque in both directions.

Incidently, why do you want a F/R switch on belt sander?

So he can back it over the line at the Belt Sander Drag Races?

(Surprisingly, the Ozito's do quite well at them. DAMHIKT. )

Actually, with the flames shooting out as it is backed up, that could look quite impressive at the belt sander races. Add a rubber belt instead of a sanding belt, put some oil down, and you could smoke the tyres, too....????

Dear Chaps,

Originally Posted by Jim Carroll

Machines like sanders are single direction and the lead in the windings gives more torque in one direction.

Originally Posted by malb

So why does it run rough and arc a lot when you do this?
The armature is 'timed' i.e. the armature windings are advanced part of a segment relative to the comutator segments to maximise torque and minimise arcing.

To make it switchable F/R, then a neutral timed armature can be used, but at the expense of an increase in arcing and a reduction in torque in both directions.

That's it guys! It's that spiral "wind-up" of the wiring from the Commutator that you can see in the second photo of my original post...

Makes sense, too. Well, that's that then, as they say. C'est La'Vie...

Originally Posted by malb

Incidently, why do you want a F/R switch on belt sander?

Malb, I've got quite a few external mitred corners in some Skirting that I want to sand back with varying degrees of "bias" to one or the other of the two intersecting pieces in order to get them to apex at their absolute join-lines. Because I need to sand away from the corner in both directions (in order to not splinter the end of the other intersecting piece), and because I can only get the sanding belt to run out to the edge of the Sander on the non-belt-drive side (which must be pointing down to get to the bottom of the Skirting), I'd therefore like to be able to reverse the direction of the sanding belt because the belt-drive-cover must obviously always point upwards (Hope that makes sense...)

Originally Posted by Skew ChiDAMN!!

So he can back it over the line at the Belt Sander Drag Races? (Surprisingly, the Ozito's do quite well at them. DAMHIKT. )

That is ultra,ultra-sharp Skew - especially as there was only three or four minutes between Malb and you!

Originally Posted by Master Splinter

Actually, with the flames shooting out as it is backed up, that could look quite impressive at the belt sander races. Add a rubber belt instead of a sanding belt, put some oil down, and you could smoke the tyres, too....????

That's pretty sharp too Splinter! You guys are Men On Fire tonight!..

Many Thanks,
Batpig.

Originally Posted by Batpig

Malb, I've got quite a few external mitred corners in some Skirting that I want to sand back with varying degrees of "bias" to one or the other of the two intersecting pieces in order to get them to apex at their absolute join-lines. Because I need to sand away from the corner in both directions (in order to not splinter the end of the other intersecting piece), and because I can only get the sanding belt to run out to the edge of the Sander on the non-belt-drive side (which must be pointing down to get to the bottom of the Skirting), I'd therefore like to be able to reverse the direction of the sanding belt because the belt-drive-cover must obviously always point upwards (Hope that makes sense...

and if you do succeed in reversing the motor, you will also have to reverse the belt.
Sanding belts are designed to run only one way — it's on account of how the join is made
reversing the motor direction will likely result in a standard belt breaking within a few seconds

Originally Posted by ian

and if you do succeed in reversing the motor, you will also have to reverse the belt.
Sanding belts are designed to run only one way — it's on account of how the join is made
reversing the motor direction will likely result in a standard belt breaking within a few seconds

I also used to believe that belts will break if used in reverse. I have been reversing my belts for years to clean them with that rubberised stick stuff. Now and again I'd leave them on reversed and noticed they never broke when used in this was and so I have been using them in either direction and cannot ever recall breaking one.

Batpig, can you just get away using an orbital or random orbital in those tight spots?

The direction of Universal "series" motors can be reversed.

You can do this by swapping the leads to the armature or the magnetic fields but not both.

The reason why you are seeing lots of sparks when the motor is reversed is because the brushes are "going against their grain". If you run the motor in reverse for a long period of time & preferably with a load, the sparks will soon disappear. After you've done this, it would be advisable to "clean" the commutator with a piece of fine grade emery cloth, until the commutator is shiny. When you've done this, give it a wipe over with a clean rag soaked in alcohol (to remove the filings).

Originally Posted by BobL

I also used to believe that belts will break if used in reverse. I have been reversing my belts for years to clean them with that rubberised stick stuff. Now and again I'd leave them on reversed and noticed they never broke when used in this was and so I have been using them in either direction and cannot ever recall breaking one.

Same here. Reversing the belt gives you more "sharp" grit, so you get an extra 15-20% life out of it. And they have never broken on me either. Its the way to go. And when sanding tops etc, I hold my sander back to front and sand into the belts direction, much like a drum sander. I get better results that way.

Dear Guys,

Originally Posted by ian

and if you do succeed in reversing the motor, you will also have to reverse the belt.

As you would well know, that is definitely doable within a few seconds Ian.

Originally Posted by Claw Hama

Batpig, can you just get away using an orbital or random orbital in those tight spots?

Maybe Claw, maybe... But the "action" is not uniquely in the preferred direction away from the external corners. But Beggars can't be Choosers, as they say. A straight Orbital would be the better of the two you suggest, because the edge of the backing-plates are straight and usually made of metal, compared to the more vulnerable round rubber that is on the bottom of my Mak ROS.

Originally Posted by elkangorito

The reason why you are seeing lots of sparks when the motor is reversed is because the brushes are "going against their grain". If you run the motor in reverse for a long period of time & preferably with a load, the sparks will soon disappear.

I know what you're saying El', and I have persevered a bit to see whether the spark situation in reverse has been abating, but I do think that the pronounced spiral wind-up of the Armature leads mentioned previously is a dominant and limiting factor. I have noticed that my Drill is both a little slower, as well as a little "sparkier", in reverse, and it doesn't even have as much "wind-up" in the leads to the coils that are running from its Commutator as the Sander has. I'm playing it by ear, though. I have to be careful that I don't melt the Sander's housing before I "bed" its brushes into reverse-direction use...

Many Thanks everyone,
Batpig.

I had a fair bit of time as a metal polisher working with belts up to150mm wide x 3.6m long in a wide variety of grits and abrasive. Most of these belts are marked with an arrow to indicate run direction, but this optional rather than essential.

In metal polishing, the belt is swapped frequently as you work through the belt grades, and the arrow helps to ensure that you mount the belt in the same direction as it previously was, or conversely in the opposite direction. Why? To get max life from a belt you run it through a fair portion of it usefull life in one direction, then once it starts to 'go off'' , reversing the rotation gives access to the back edges of the abrasive grains, and extends belt life by about 35 to 40% over single direction only use.

Probably saw an average of one belt actually break out of maybe 150 worked to death in a week, but these were normally due to to the belt snagging or being cut by the workpiece, rather than the joint failing. Quite interesting to see a 3.6m belt thrash itself to death driven by a 5HP motor with people trying to dodge the flying strands and stop the machine.

Originally Posted by Batpig

...asks...am I missing something fundamentally simple?...

Hi Batpig,

COming from a former industrial engineer, while it's possible to reverse direction of the motor, the threads on all the shafts, etc... are designed to tighten under load.

It's probable that if you reverse the direction of the motor, you'll start undoing things unintentionally.

That is, unless every connection is splined/keyed into place.

Cheers,

eddie

Hi Michael,

you'll wreck the sander for sure, so don't attempt this. The optimisation of the armature coils and the resulting magnetic field, which is rendered into a time-lag when used in reverse, is mentioned as an argument against doing this several times in the other posts, and rightly so. But all other things go in reverse too, like the drive train, the belt system, the cooling air flow and the dust suction fan.

For instance, when you look at the cooling fan mounted on the armature, you see the blades curved in a certain way. That means they are optimised for scooping the air from the direction of the commutator and the rotor coils towards the fan itself. Run in reverse, curved blades only produce a fraction of the designed and calculated airflow. Forthermore, the air is supposed to first cool the commutator and then the coils. If the air is heated up after passing the coils, the commutator will run too hot and there is already much more sparking then this system was designed for in the first place.

As for the sanding belt run in reverse, think of a chain saw run in reverse. Normally the chain is drawn through the wood by the motor driven sprocket. Only the blade's bottom part and the sprocket take the strain. The blade's top part and the star wheel in front are meant to guide the chain round and back to the sawing process again. Experienced users are seen to use the blade's top part as well, to saw little branches from felled trees in very quick expert movements. But they know that the machine can not take too much strain this way, because the pulling force is transfered from the sprocket to the star wheel, which has a very flimsy to an almost non-existing bearing system.
On a belt sander run in reverse, the pulling takes place at the upper belt part and the pulling strain on the sanding bottom part of the belt sander is transfered from the hind rubber coated drive roller to the front guide roller. This roller has a decent bearing system and it can take this strain, and traction from the hind roller will take place just as good whith this roller as an inbetween. But the guide roller has a steering function, taking care of the proper belt track. With pulling force on this steering roller, the belt tracking will probably go wrong very quickly and adjusting will be very finnicky.

In reverse the dust will not be drawn away at the back by the suction fan, but spewed out from underneath the belt at the front. The fan itself will not work properly, either. Centrifugal fans do not blow instead of suck when rotated in reverse, but the shape of their enclosure determines that their designed efficiency works in one direction only. Run the other way, the displaced amount of air may be 30% or less of nominal output.

The drive train is probably a toothed belt as a primary step, and a set of gears as a secondary step to drive the hind roller. The toothed belt will run fine in both directions, in worst case a pulley may loosen itself when there is a thread involved for fixing or securing. The gears, however, may be ground helical. There is always a slight axial thrust component generated by helical systemes, which is absorbed by some of the bearings. This axial thrust will point in opposite direction when such gears run in reverse, which may be harmful -or not. I don't know your type and make of machine, so i can't determine if it is or if it ain't..

Sanding belts are cut from large rolls of fortified linen strips, carrying sprinkled and glued abrasive grains. At both ready cut glue joints of a length of strip, the grains are rubbed off, in order to have a dependable adhesive contact between fabric and fabric. The fabric overlaps on this joint and the inside overlap must point in the rotating direction of the belt. When the rubber coated drive roller pulls at the joint, the overlap will take the strain without fraying, when pointing in the right direction. With the belt mounted the wrong way, the pulling rubber of the drive roller may peel off the glued overlap bit by bit at every loop pass, which may eventually make the sanding belt snap or tear. So there's more involved than just a reversing switch, running in reverse also means having to mount the belt in reverse.

And than there's the base plate, which is often a heat treated steel plate or some sort of graphite coated fabric. It is fixed at the leading edge only, to allow heat expansion and shrinkage at the trailing edge. With the sanding belt running in reverse, a graphite coated piece of fabric would just crumple and tear loose. But with a steel plate, the belt could snag behind its loose trailing edge, with sure damage to the belt and plate and possible damage to your health. Since the inside joint overlap of the sanding belt must point in the running direction and will collide head on with the loose trailing edge of the plate when run in reverse, chances on snagging are big.

So, all in all, as said in the other contributions above, the disadvantages vastly outnumber the advantage, so do not invest in the effort.

greetings!

gerhard