Most Motorized or High Frequency spindles are designed to be cooled by liquid. Otherwise the heat created in the motor will cause the bearings to run hot and eventually lead to catastrophic spindle failure. Rule of thumb is to have 1 BTU/hr of cooling capacity per motor HP. This is accomplished with a Spindle Cooling System. Liquid is cooled and pumped through a designated closed loop inside the spindle, cooling the motor and in some cases even the bearing housings.
The liquid used has to be filtered to prevent blockage of the system over time. It also needs to be of a non corrosive nature so the steel, seals and O-rings used in the spindle are not affected. It is usually a water/oil mix with additives such as rust inhibitors, algaecides and/or glycol. The more oil is in the water the lower the cooling properties will be.
The Coolant Temperature should be kept within 0 to +5 Degrees of ambient to prevent condensation and possibly the decrease of the internal bearing clearance. Decreasing this clearance from outer race to inner race will result in over pre-loading the bearing which will then create additional heat… the vicious cycle has begun, the result will be spindle failure!! So… TOO COOL is not cool but deadly!