The fit of the carbon portion of the brush in the holder is critical for stable electrical contact. If there is inadequate space between the holder walls and the thickness and width of the brush there is potential for binding of the brush in the holder particularly with increased temperature and contamination. On the other hand an excess amount of space between the holder and the carbon will result in an unstable electrical contact as the brush face can move tangentially or axially within the holder.
The holder and brush tolerances on the thickness and width therefore must be well coordinated. Brushes are machined undersize per NEMA tolerances or per drawing specifications while brushholders are made oversize. As a general guideline for brushholders, industrial sizes typically should be held oversize to a tolerance of +.002/+.008”. Smaller frame units with a brush thickness less than .500” and greater than .125” should have holders with a tolerance of +.001/+.005”. Micro size units with brushes of thickness .125” or less should have holders held to a tolerance of +.001/+.003”.
Over a long period of usage the thickness dimension on a holder can become worn from brush movement or distorted from heat. Therefore it is important to periodically measure the thickness and width dimensions on the top and bottom of the holders to ensure they are within tolerance and that the brush will have adequate support for a stable electrical contact.
When motor and generator brushholders are subjected to high temperatures, it may be necessary to provide extra compensation for thermal expansion depending on the temperature rise and the degree of heat dissipation. In these cases it is easier to reduce the brush thickness and width dimensions slightly to avoid sticking in the holder rather than adjusting holder dimensions. Metal graphite brushes with over 50% metal content by weight are manufactured with an increased undersize tolerance per NEMA standards as they usually carry higher current, generate more heat, and have a higher coefficient of thermal expansion than non-metal grades.
Brush and holder length can also have a significant effect on the stability and performance of the brush. Most often the length is limited due to the space available within the frame. There are however also practical length limitations due to the excess resistance of a long piece of carbon. As the carbon length is increased the resistance of the current path from the shunt to the contact surface is increased. At the same time the amount of contact area between the carbon and the longer holder is increased and the corresponding contact resistance is decreased. This then creates the potential for distorted current flow directly between the holder and the carbon rather than through the shunt.
On the other hand short brush and holder designs are more susceptible to instability at the contact surface. There is potential for a higher degree of brush tilt in the holder since the length of support is less in relation to the brush thickness.
In addition to dimensional concerns the insides of the holder must be smooth and free of all obstructions including burrs. If a used brush has any straight scratches down the sides of the carbon then there are protrusions inside the brush box, which will restrict the brush from making proper electrical contact. Rough handling of brushholders can cause distortion of the metal and effect the critical inside dimensions of the brush cavity. Holders made from metal stampings are particularly susceptible to irregularities on the inside dimensions and on squareness. Broaching is generally accepted as the best manufacturing method for assurance of consistent inside dimensions and a smooth finish.
Contact Team Phynyx for inquiries or consultation
To get free consultation or an estimate for a custom carbon brush or carbon brush holder please email us at sales@phynyxind.com