Hedge Trimmer Brushes

[Hysteresis]

I have a GardenLine (B&Q) hedge Trimmer. It has worked for a few years but suddenly stopped.
I found the commutator brushed worn down so after some problems identifying a supply of replacement brushes I finally bought some from the Machine distributers. Replaced the brushes and all went good. After three minutes use, the machine slowed down, so I stopped it and noticed a bit of blue smoke coming from the Commutator vent. Stripped the machine down and found the brushes half worn down. I had previously cleaned the commutator with fine sand paper and noted that no undercutting was needed, After this three minutes running the commutator was blackened with Carbon from the brushes and after inspecting the commutator the segments were not heat discoloured so I assume the armature to be OK. I am at a loss now for any ideas as to what is causing rapid brush wear and subsequent failure.
The machine runs OK so there is no Motor problem I can Identify. The mechanical drive from the motor to the cutter heads are free and not obstructed so no mechanical loading problems.
Any Ideas from Anyone as to what the problem may be.

 

[Hysteresis]

As a supplement to the question raised above, I have made a jig to hold the brushes in place so that I can observe the commutation of the motor with the machine housing cover removed. I observe excessive sparking at the brushes. As mentioned earlier, the commutator is clean and undercut, so what else can cause this excessive commutation sparking. One can theorise about neutral axis problems but this is all down to fundamental machine design and not the issue here. The motor works, the problem is excessive commutation sparking. Given that the fundamental design of the machine is unchanged, there must be another component failure which has created the problem but I am not knowledgeable enough to identify the possible component responsible.?

 

[conny]

Soft brush grade? You probably know carbon brushes come in different grades, maybe the ones you have bought are too soft. You will need to know what the original grade were and order some of them. Obviously if you get a harder grade they will wear the comm segs down but certainly not as quickly as the soft brushes wear.
When you cleaned the comm previously did you use the glasspaper one handed or did you use a narrow strip, pulling it back and forth around the circumference of the comm before rotating and doing the next section? On a small comm it should only take 3 turns of the comm to cover all the segs. You also need to brush/blow all the carbon dust away before bedding the new brushes in to the shape of the comm.

 

[Hysteresis]

Thanks for your comments conny, the brushes were supplied by the same company that sells the trimer, they were identical to the original so assume they are of the hardness of the original brushes. I cleaned the com by using glass paper and rotated the com using the shaft on the opposite end of the armature, radial cleaning therefor assuring no flats on the com. After attending to the com the dust was both brushed and blown to clean it out. The rate at which the new brushes wore down was dramatic at half the length of the new brush in about 3 minutes run time which lead me down the track of looking for a loose com segment, but all seemed as designed. There must be something mechanical affecting the brushes but I can detect anything at all that would 'grind' the brushes so violently. Maybe it is a displace com segment that I can't identify, if that is the problem it is uneconomical to repair.
I am convinced my problem is mechanical but just can't locate any mechanical issue. I think the trimmer is destined for the scrap yard, I have spent quite a long time trying to sort it out, and bought a new set of brushes at an exorbitant cost. A new trimmer will be the answer I fear and cut my losses in time and money..