Why is a PMDC motor getting hard, when on load after 10 years of service?

[Hobbyist]

Why is a PMDC motor getting hard, when on load after 10 years of service?

 

[Hobbyist]

Yes, there is a little carbon build up.

The bearings may be wearing out. Even if the motor is not stiff in itself, if the drive end carries a gear and the bearing is worn so that it does not mesh accurately, the wrong surfaces on the gear will rub together and cause friction.

Does the motor work correctly on power? An internal short-circuit in the armature, or between commutator segments, can make the motor stiff to turn. A PM motor works equally as a generator and any short will allow current to flow within the armature and create a torque opposing rotation. If the gearbox has a high reduction ratio it might be important that it spins very freely, in which case perhaps even a carbon build-up between the commutator segments will pass enough current to create too much dynamic braking.

"I have replaced the bearings and will now check after 2 - 3 days if it works well on the test module i have, in the meantime i wanted to know if any other factors is responsible for it's hardness like weak magnetism etc."
Posted the above 2 days ago.
Status now: no change in pulling force even after replacement of bearings
Guide me for any other factors...

 

[Lucien Nunes]

How does the shaft couple to the gearbox - could it be a mechanical alignment issue that is causing a little extra friction even though you have changed the bearings? Can you post a picture of one of the motors so we can understand the mechanical setup?

Do try cleaning the slots between the commutator segments and then polishing the comm, even if only to rule that out. There isn't much else, weak magnetism would cause it to run more freely as the cogging (reluctance) torque will be lower.

You might like to compare the current both on and off load between a new and old motor. That would be quite revealing.

 

[Hobbyist]

How does the shaft couple to the gearbox - could it be a mechanical alignment issue that is causing a little extra friction even though you have changed the bearings? Can you post a picture of one of the motors so we can understand the mechanical setup?

Do try cleaning the slots between the commutator segments and then polishing the comm, even if only to rule that out. There isn't much else, weak magnetism would cause it to run more freely as the cogging (reluctance) torque will be lower.

You might like to compare the current both on and off load between a new and old motor. That would be quite revealing.

 

[Hobbyist]

I have literally pushed the carbon brushes out to rule out it's hindrance (if any). I will check the operation and revert back.

 

[Lucien Nunes]

Is the driving gear of the planetary gearbox attached to the motor armature? Is it plastic or metal? Are you swapping armatures relative to the rest of the gearbox or keeping them as units? Does a new motor armature run freely with an old planetary gearbox?

I am thinking that the gear on the old motor is worn and causing extra friction, or there is wear or play in the gearbox that makes it intolerant of all but a new motor gear. You mentioned replacing the bearings - that's both bearings on the motor shaft. correct?

I would definitely try the current comparison.

 

[Hobbyist]

I have literally pushed the carbon brushes out to rule out it's hindrance (if any). I will check the operation and revert back.

status remains the same

 

[Hobbyist]

I have literally pushed the carbon brushes out to rule out it's hindrance (if any). I will check the operation and revert back.

Status remains the same even after putting the carbon brush out of the way of armature.

 

[Hobbyist]

Is the driving gear of the planetary gearbox attached to the motor armature? Is it plastic or metal? Are you swapping armatures relative to the rest of the gearbox or keeping them as units? Does a new motor armature run freely with an old planetary gearbox?

I am thinking that the gear on the old motor is worn and causing extra friction, or there is wear or play in the gearbox that makes it intolerant of all but a new motor gear. You mentioned replacing the bearings - that's both bearings on the motor shaft. correct?

I would definitely try the current comparison.

Yes, the driving gear of the planetary gearbox, i.e. the white plastic spur gears are attached to the motor armature via. the shaft of the motor. Yes, correct i have replaced both the bearings.

 

[Hobbyist]

Is the driving gear of the planetary gearbox attached to the motor armature? Is it plastic or metal? Are you swapping armatures relative to the rest of the gearbox or keeping them as units? Does a new motor armature run freely with an old planetary gearbox?

I am thinking that the gear on the old motor is worn and causing extra friction, or there is wear or play in the gearbox that makes it intolerant of all but a new motor gear. You mentioned replacing the bearings - that's both bearings on the motor shaft. correct?

I would definitely try the current comparison.

Is the driving gear of the planetary gearbox attached to the motor armature? Is it plastic or metal? Are you swapping armatures relative to the rest of the gearbox or keeping them as units? Does a new motor armature run freely with an old planetary gearbox?

I am thinking that the gear on the old motor is worn and causing extra friction, or there is wear or play in the gearbox that makes it intolerant of all but a new motor gear. You mentioned replacing the bearings - that's both bearings on the motor shaft. correct?

I would definitely try the current comparison.

When i pull the door with a new motor and the old gear assembly (decoupled from the faulty motor), everything is fine. However, when i pull the door with the faulty motor and a new gear assembly (decoupled from a spare new motor), the pull becomes hard. Hence, i can conclude that it is something inside the motor that is making the operation hard.