Spindle
Accessories
The spindle should be constantly monitored: Installing SMRU
It is not a very easy task to properly maintain spindles because they are manufactured in such a way so that they can work in a certain temperature range for a certain period in time. Different temperature range has different effects on the spindle. Some experts have suggested the installing of a spindle monitoring and recording unit (SMRU). This unit provides an easy way to keep track of critical maintenance issues. For example, when operated at 15°C above normal, installing of SMRU will give an indication that grease life decreases by a factor of 2. SMRUs are used to record temperature, running hours and speed. Another version of SMRU is that the information is stored in a histogram form which helps in providing the maintenance engineer with information to find out how long the spindle has been running and also to predict the remaining grease life. This unit can be installed as a part of the spindle or bolted to it. Usually software is available for configuration of the SMRUs displays and settings.
Ceramics are better
For a much better performance of spindles, try to replace a spindle's bearings with hybrid ceramic bearings. For example, in a grinding spindles, 25 percent higher rpm can be achieved by using hybrid ceramic bearings and synthetic grease lubricant in place of steel ball bearings using oil or air-mist running at 10,000 to 15,000 rpm. Grinders with ceramic bearings can run 4000 hours without problems, when compared to 3000 hours with steel bearings. There are hybrid bearings running up to 5000 hours at speeds more than 2.0 million ndm before degradation of parts occurs. Ndm is defined as the bearing pitch diameter in mm multiplied by rpm). On the other hand, a steel bearing can quit after 700 hours. Hybrid ceramic bearings also have the benefit of lowering temperature almost 50 percent and are more durable than steel bearings. In any machining tool center, the use of hybrid ceramics bearings has been shown to drop bearing temperature from 60°C to 36°C at 12000 rpm.
Sophisticated belt drives
Another factor that can be considered to improve the performance of spindles is to use sophisticated belt drives rather than conventional belt-driven spindles. A normal belt drive uses a angular contact bearings pair at the belt-drive end and studies have shown that such belt drive face many problems with spindle deflection and premature failures. At very high speeds, the bearings are loaded radially with the belt pull and it also restricts the axial movement of the bearings which compensates the thermal growth of the spindle system.
Shielded ball bearings
To remove the need for air purge or spot oil lubrication, it is necessary to upgrading the shielded ball bearings. This can improve the spindle's performance because it can reduce contamination and helps to increase spindle speed. A standard or a normal shielded bearings have the same ball diameter, speed rating, internal geometry, load ratings, matching criteria, preload classes, and contact angles as the bearings they replace. On the other hand shielded bearings are dependent only on the pre-packed lubricant of the bearing. A double-shielded design has smaller diameter rolling elements and the rings have open conformities for high speed performance. As such it has higher speed capability but lower load ratings compared to the same size of the standard bearing.
Use of air seals
Air seals contain usually an air distribution ring and a rotating metal cased elastic sealing which have the advantages of contact seals and air purge. Air seals are provided by few manufacturers in spindles. The elastic sealing move up from the contact surface as the rotation of the spindle increases the centrifugal force. The metal case cast away any kind of contaminants coming into the spindle. There is uniform airflow from the air-distribution ring. This helps the seal to move on a thin film of air while rotating. The air also pushes contaminants and dust away from the spindle while moving. In a static condition also, the elastic seal provides a positive contact by keeping away contaminants from entering into the spindle.
Some Do's and Dont's
Since the spindle is the heart of the machine, it must always be handled with care and maintained regularly for best performance. The following list provides few easy checks you can make to reduce your spindle repair cost and improve the spindle's performance.
- If the spindle is kept in a storage without using it for some time,
then rotate the shaft 3 or more turns quarterly top to bottom to
circulate grease and lubricate internal components.
- Store the spindle in a dry heated area and in an area away from
traffic to avoid damage.
- Keep the spindle covered covered so as to avoid dust and other
foreign matter if not in use.
- Do not bump or stand on pulley ends while handling or installation.
- Check the spindle mounting surfaces for flatness and remove all
dirt.
- Change filters before connecting them to the spindle.
- Flush out lubrication and air purge supply lines.
- Check housing for leaks and tears.
- Check for a clean lubricant supply.
- Check for a correct operating pressure.
- Check the air purge operating pressure and flow.
- Check pulley and belt for operating and do not tighten the belt very
heavily.
- Check spindle rotation direction and speed.
- Use balanced tooling assemblies and cutting tools.
- Check the tool clamping pressure.
- Do not blow air or coolant at the face of spindle or spindle seals.
- If your spindle needs repair service, then always ship all spindle
components, keys, pulleys so they can be checked for proper balance.
- Clean the spindle's exterior before shipping it to the repair house etc.
