Stepper motor torque problems?

[Chris D]

My stepper motors appear to be getting weaker over time. My step is this...

40 volts ->
HobbyCNC chopper driver ->
5 volts, 1.4 amp, 160 Oz-In motors ->
.2" pitch ball screws.

I used to get about 90 pounds of force when pushing against a scale at a low feedrate. Now it is down to around 40 pounds of force (the machine has been in use for about a year).

Voltages, current, cooling, screws, ways, etc., all are correctly set and working freely and properly. The motors run fairly cool at about 120 degrees, the driver chips run around 125 degrees on the HobbyCNC board.

Anyone have any ideas as to why the torque on the motors seems to be dropping?

TIA

Chris

[Al_The_Man]

Everthing else being equal, it can be caused by de-magnetization of the motors, generally this is caused by shock or high spike currents. A way to test them is if you have the holding torque rating is to apply the exact DC rated voltage to the winding and measure the holding torque by Lever arm and spring balance method.
The other measurment is to see if your DC collapses too much under load.
Al.

[Chris D]

Thanks for the info Al,

I am suspecting de-mag as the problem, but one of the three motors has ALWAYS run about 25 degrees hotter than the other two. This motor is also the weakest of the 3 at this time.

I have a new 200 Oz-In motor comming and when I pull things apart I can do some checks.

How would I measure to check the DC collapse?

Thanks

Chris

[Al_The_Man]

While drawing a constant high load, you can measure it with a meter across the DC supply.
Al.

[James Newton]

Is it true that motor heat and the spikes that come as a part of any chopper design can cause this? How can you tell if it is happening and protect against it?