How to maintain your robots

Mike Howard, service manager of Automated Assemblies Corp. (Clinton, MA), offers some good technical tips for preventive robot maintenance in a recent edition of AAC's "Robotic Resource" newsletter. Howard begins by saying that the days of "one lubricant, grease on the fly, and fill till you see the grease appear" are over. These days, controls are too intricate, mechanical components are too precise, and robot electronics are too sophisticated for such old-fashioned maintenance methods. After all, robot makers have had to keep pace with your increasing demands to improve the operational capabilities of their products. Consequently, these greater demands on them demand an improvement in the skills of your maintenance staff.

Different mechanical items in robots require different lubricants, schedules, and amounts. "The use of the correct lubricant is very important and depends on the action of the device it is being used on," Howard says. Weight, speed, load, environment, temperature, and mechanical operation are among the criteria robot OEMs consider when recommending a particular grease or oil, or maintenance procedure. Howard and company use cycle time, travel, and mechanical specs as the basis for the following maintenance guidelines for four key robot systems.

Linear Bearings/Cam Followers/Rails

Linear bearings and rails must be periodically wiped down and inspected. Lubricants specific to the linear bearings are required. Over-lubrication will cause dust wipers on the bearing blocks to expand. This expansion can allow contaminants to find their way into the bearings. "The fastest way to destroy the bearings is by not lubricating them," Howard warns. "The bearings will burn, bind, drag, and cause the motor mechanisms to operate harder. The closely engineered tolerances will be lost, resulting in loss of repeatability and excessive play."

An OEM-prescribed maintenance procedure also must be followed for cam followers and their cam rails. Howard says that cam followers use needle bearings captured within a cage holding them in place, and that incorrect lubricants will cause the needle bearing to wear and burn because of friction. This locks up the roller and causes it to skid along the rail. Overpacking the cam follower will blow the seals, which, again, will allow the entry of contaminants.

Ball Screws/Rack and Gear/Belts and Pulleys

To extend robots' service life and to maintain designed contact tolerances, ball screws and their bearing knuckles need to be cleaned and lubricated. Rack and gear mechanisms have an engagement tolerance that needs to be held. Belts and pulleys must be tracked and set for proper tension. And, Howard says, the motor mechanisms these devices are connected to have to be tuned to these specifications and loads. "Variations resulting from improper or nonexistent maintenance practices will be evident with the loss of repeatability, unusual noise, and movement alarms."

Pneumatic Cylinders/Valves/Filters

Pneumatic devices, driven as they are by air pressure, must be as oil-free and as water-free as possible. Heat, water, oil, dirt, and, to some degree, acid residue are produced through the action of compressing air. If left unchecked, and improperly cleaned, dehumidified, and cooled, these air contaminants can work their way into components like cylinders and valves, break down seals, and cause valves to stick. "The robot has been supplied with an air regulator and filter," Howard says. "This needs attention and cleaning periodically."

Motors/Controls

In and of themselves, Howard says motors tend to be very resilient, requiring only minimal care. But problems can be caused by uncontrolled power fluctuations, such as blowouts or direct shorts to their windings. Cabling needs attention as to how and where it may be run. That's because electronic noise generated by welders or other inductive leads can introduce pulses to the resolver or encoder cables, affecting motor positioning.

Control cabinets are insulated and offer protection to the PLC and various other control components. Fans and filters are provided for the additional cooling and particle filtration needed. Heat buildup can affect operation, as can dust, a great electrical conductor, if the filters are not cleaned, or if they are removed for some reason. Items such as interface relays should be replaced yearly.

Howard concludes by saying that scheduling maintenance downtime is a must. "All components have life cycles. The greatest variable then becomes the type and frequency of the maintenance performed. The attitude of 'run 'til it drops' will only come back to compound itself 10 times over." Components fail for one reason or another, so check the spare parts list in your robot operations manual, and make sure you have an adequate inventory of spares.