Grinder: Motor Gets Hot

The armature, located inside the field, is the power-producing unit of the motor. Common causes of a hot running armature are due to heavy dirt build up on the inside, a broken cooling fan, cooling ports blocked – leading to poor air circulation to keep cool – or working in hot environments, overloading the motor. It also could result from excessive sparks from the brushes which can cause the commutator to overheat, transferring the heat to the armature. When the armature becomes too hot, the insulation on the windings can break down and cause the armature to ground out.

To remove the armature to check and clean it, remove the handle and wheel cover then remove the gear housing from the grinder. Remove the brush holders by removing the rear cover to gain access to the brush holders. Some models may require you to remove the paddle and lock button as inside caps, which are located on the side of the grinder. It can be removed easily with a screwdriver.

Remove the armature from the grinder and clean it with electrical contact cleaner. Inspect for any burn marks, indicating a short, and replace if there are any. Check the commutator for any missing bars and replace the armature if bars are missing. For further instructions on replacing the armature, follow instructions further down. Clean the commutator with 600-grit aluminum oxide sandpaper only in the direction of travel, to avoid damage. Use a plastic scribe to clean out any debris in between the bars on the commutator that can cause a short.

Next, test it for continuity to make sure the armature is in good working order. To check for a bad armature, you can perform the 180-degree test. With the armature removed from the unit, stand it up on a bench. With an ohmmeter, attach the two ends of it to both sides of the commutator to measure the resistance of the windings. As you move it around, the reading number is not important. You are looking for a consistency in reading, indicating all the windings are good. If it varies radically, reading zero or open circuit, then the armature is not good.

Another test is reading. If the ohmmeter reads zero, or open circuit, then that indicates a bad armature. The last test is a bar-to-ground test, which is done by placing one lead of the ohmmeter to the tip of the shaft of the armature and the other end of the ohmmeter to the commutator. If the armature is bad, install a new armature.

Remove the nut securing the armature gear and take out the gear. To remove the gear box cover and bearing, place the cover on top of a bench vice, opened enough for the cover to sit on. Tap the top of the armature shaft with a rubber mallet to remove the bearing and cover. Install the cover and bearing onto the new armature and use a socket big enough that it only makes contact with the bearing race to avoid damage. Tap in place with a hammer.

Reinstall the armature gear and reinstall the nut to secure it in place. Reinstall the armature back into the grinder. Reattach the gear box assembly to the grinder. Reinstall the brushes and reapply the tension springs and lower cover. If applicable, reattach the paddle and lock button at this point. Finally, reattach the handle and wheel cover.

A seized bearing on the armature shaft will create a lot of excessive heat in the motor. The following procedure will vary on all models. To inspect for a possible seized bearing, remove the grinding wheel cover and handle. Remove the outer flange, grinder wheel and inner flange. Remove the screws securing the gear box assembly to the grinder and then separate the gear box assembly from the grinder. Remove the lower cover. Some models may require you to remove a paddle and lock button. Pull the tension springs on the brushes to the side and take out the brushes.

On other models, remove the brush caps to remove the carbon brushes. Remove the gear housing cover and pull the armature assembly out of the grinder. Inspect the armature bearings by spinning them with your hand. If you find one that will not spin, that means it is seized.

For the front armature bearing, remove the armature gear. Use a bearing separator, or puller, to remove the bearing. Install the new bearing by placing it on the shaft then use a socket that makes contact with the inner race of the bearing to avoid damaging the bearing. Tap it in place with a hammer. Reinstall the armature gear with the nut, or retaining ring, that holds the gear in place.

To replace the lower armature bearing, remove it with a bearing separator, or a puller, and then install the new bearing on the shaft using a socket that only makes contact with the inner race of the bearing to avoid damage. Tap in place with a hammer. Reinstall the armature assembly back into the grinder. Install the gear case cover and the gear box assembly. Reinstall the brushes and reapply the tension springs back onto the brushes. Reattach the lower cover and, if applicable, the paddle and the lock button. Reinstall the grinding wheel cover and handle.

The carbon brushes supply electricity to the armature by rotating on the commutator. As the carbon brushes wear down, become worn and with the build up of dirt and debris, they will draw more current and cause the motor to get hot as the brushes spark more. To inspect the brushes, remove the lower cover on the grinder, locate the brushes and then release the tension springs to remove the brushes.

With the brushes removed, inspect them for uneven wear, damage and weak springs. Any of these symptoms will cause the motor to run hotter than normal. Brushes getting close to an 1/8 of an inch, or less, need to be replaced. If the brushes are in good shape, then clean them off lightly with some sandpaper. Clean any dirt out of the holders with electrical contact cleaner and install the new brushes, or cleaned brushes, back into the holders. Reapply the tension spring to hold them in place and reinstall the bottom cover on the grinder. On some models, the brush caps are conveniently located on the side of the grinder for easy access. Remove these caps with a screwdriver to inspect the brushes. Reinsert the brushes back into the caps then reinstall into the grinder.

The fans keep the motor cool as well as all the other internal parts of the grinder that generates heat. A broken fan will not be able to circulate adequate air to prevent overheating of the motor. Different models have the cooling fan located in different spots, so repair procedures will vary.

Some models have the fan located at the front of the armature shaft and, in most cases, is sold as an armature assembly unit replacement only. Refer to “armature replacement” for this repair procedure.

Other models have the cooling fan located in the rear of the machine on the armature shaft. To replace the fan, remove the side cover on the lower part of the grinder. Remove the paddle and the lock button and then take off the other half of the side cover. With the fan exposed, push the lock pin down to lock the motor in place and unscrew the fan. The fan is reverse thread on the shaft. Install the new fan onto the shaft and then reinstall the side cover, the lock button and the paddle back onto the grinder. Tuck all the wires back into their appropriate channels and reinstall the other side cover, making sure everything lines up.

The field is an electromagnet used to generate a magnetic field in an electro-magnetic machine. Over time, this part can build up a lot of heavy dirt and cause excessive heat in the motor as the dirt will not allow the heat to dissipate quickly. This is serious; if left unattended, it will lead to more severe problems such as shorting out, smoke and failure of the field.

To inspect the field for burn marks, indicating excessive sparking, you will need to remove it from the grinder. To do this, start by removing the gear box assembly from the grinder. Remove the lower cover and remove the tension springs from the brushes and then pull the brushes out of the brush holders. There may also be a paddle and lock button to remove, as well as a second side cover.

For some models, you will have to remove the brush caps to remove the brushes. Pull the armature out of the field and remove the baffle plate. Then, remove the power switch by holding it in the off position and pressing the switch lever forward. When you hear the click, you can remove the power button. Disconnect the wire connections and remember where they attach to for reassembly. Remove the field from the grinder housing. You may have to hang the housing on a table in order to free the field as it may be secured tight. Clean the field with electrical contact cleaner and then inspect the field for burn marks, indicating sparking and shorting out. If you notice burn marks on the field, it probably needs to be replaced.

You also can perform one simple test on the field to check for continuity by using a multimeter. Set the multimeter to the ohmmeter and place a lead on each end of the field. If the reading is infinity, the field is bad and requires replacement. Install the field into the body of the grinder in the correct position for the wires to reattach to their proper connections. You may need a long screwdriver to position the wires with the opening during installation of the field.

Reconnect all of the wires back to the switch and brush holders. Push the switch lever forward and reinstall the power switch, making sure the lever on the switch catches the lever. Reinstall the baffle plate and then slide the armature back into the field. Reinstall the gear box assembly back on the grinder. Reinstall the brushes back into the field. Reinstall the gear box assembly back onto the grinder. Reattach the brushes back into the holders and then reapply the tension springs, follow by reattaching the lower cover.

Make sure the gears are lubricated. Poorly lubricated gears, or if the gears have no lubrication, will generate more heat and transfer this heat to the motor through the armature shaft. Remove the grinding wheel cover from the grinder. Take out the spindle box housing from the gear box housing. Clean out the old grease with thinner and then repack with new grease. Now, reassemble the spindle box housing to the gear box housing.

The holders house and support the springs and brushes, allowing the brushes to ride smoothly up and down on the commutator, as the commutator may not be true. Any physical damage to the holder, or a heavy build up of dirt, can impact its ability to hold equal pressure on the commutator, causing more sparks and resulting in an overheating motor.

To check the brush holder, you will have to remove them. Remove the lower cover to access the brush holders. For some models, you may need to remove the paddle and lock button as well as a second side cover. Release the tension on the brush by moving the tension spring to the side and then remove the brush. With the brush removed, move the tension spring back. Take out the electrical connections at the brush holder and then remove the screws securing the brush holder.

Inspect the brush holders for distortion or anything else out of the ordinary. Install the new brush holder and secure it with the screws. Reattach the electrical connections. Move the tension spring out of the way and install the brush. Then, apply the tension spring on top of the brush. Reinstall the electrical connection, followed by the cover.