Lathe Belt Tension Adjustment
with Main Spindle Motor

The main spindle motor has the belt drive system from main motor at the machine base of CNC lathe onto the main pulley from headstock for spindle rotation. The performance of this CNC lathe is directly related to the proper installation and maintenance of the main spindle motor belts. Improper V-belt tension is the most common operating problem for belt-driven electric motors.  

A. Locate the backside of spindle

B.Open the door to see motor belts

Proper tensioning of V-belt drives will make it possible for extended drive life. On the other side, improper tensioning leads to early failure of the driving system and can pose a safety hazard.
 
When a new CNC lathe is delivered, the tensioning of the V-Belts is properly adjusted in such a way so that the belts have a very slight deflection, say, about 1/4” (or around 6 mm) when you hold one of the belts at the center position or half way between the top pulley from the headstock and the pulley from main drive motor and push towards the center.

C. These belts in above picture are very loose which must be adjusted.

D.The cover for the main spindle motor is removed

The efficiency of the main spindle motor can be high or low depending upon how properly the adjustment is made for tensions of belts.
                                          
The tensioning of the V-Belts of the main spindle motor should be examined on a weekly basis when electric power of the machine is SHUT OFF completely.
 
When it is necessary to adjust the tension of the V-belts, one must turn OFF all the electric power to the CNC lathe first. Then, remove the sheet metal cover at the bottom portion of the left hand side of the lathe in order to have the access to the main spindle motor as shown in the picture D.
 
The main spindle motor is supported by a base plate, which position can be adjusted with two long bolts & multiple lock nuts at the left hand side of the main motor.
 
When to increase the tension of the V-belts, the entire base plate must be set into a lower position. Before making adjustment, please provide some wood blocks under the base plate at current position to support the weight of the main motor. Then, use adjustable wrench to loosen the lock nut underside the base plate (in orange color in the picture below). This step should apply onto both bolts evenly, one in front and one on the back side. After the adjustment, one must lock down the nuts on the top again on both bolts, one in front and one on the back side.   

Enlarged view of the main spindle motor (in grey color), its base plate (in orange color) and two long bolts to be used for making adjustment of the working position.