Air cylinder regulation. (Mike Breen on Preventive Maintenance).
I recently received a service request to examine an automatic tool changer (ATC) that was throwing only the heavier tools. The fault was tied to an air cylinder. The flow regulators' (mounted on the cylinder air ports) lock nuts had backed off and the needle valve slowly retracted, allowing unrestricted airflow to the cylinder.
The cylinder controlled the position of the waiting pot on the ATC assembly. The pot would grab the tool from the magazine and put the tool into position so that the ATC arm could grasp it and place it into the spindle. The cylinder was slamming so hard that the tool was popping out of the pot. When the arm came around to grab the tool, the arm would grip the tool holder in the wrong position and during its quick rotation, would unpredictably throw the tool in all directions. Unfortunately, this caused a damaged cylinder. The related mounting brackets and the "Y" axis cover (where the tool would routinely hit) also required replacement.
The same fault caused problems with a pallet changer gearbox. The pallet changer gearbox rotated the pallet changer so quickly and the weight and centrifugal force caused by the unregulated hydraulic flow caused significant damage to gears and related shafts. The repair was extremely expensive and the downtime was a week because parts had to be flown in from Japan.
Both of these incidents are the costly result of improper testing and regulation of the air and hydraulic cylinders. Faults like these would and should be caught during a preventative maintenance inspection. The hardest part about inspecting these systems (especially ATC systems) is that the newer machines' sub-cycles are getting faster and faster. This stems from the constant battle of cycle time reduction.
To check air cylinder performance, run multiple cycles and watch closely. This could take twenty tool changes to notice a slight problem and twenty more to adjust it properly. The regulators are either mounted to the cylinder or the solenoid valve that controls the directional flow of air or oil. When they're mounted to the solenoid valve, it can be difficult to determine the function of each one. Examine the flow diagram that is usually mounted to the solenoid itself and adjust it.
Don't go too far too fast. The controller times many of these cylinders, and if the actuation takes too long, an alarm may occur. Some machines can lock-up hard when the canned cycle faults at a particular sequence. Be sure that you're familiar with all sequences of the cycle prior to adjusting it. You may be forced to manually finish the cycle, either through forcing outputs through the control or using the old method of sticking an allen wrench into a solenoid to shift the spool. In any event, be sure you know exactly what you're doing. If you force the wrong component at the wrong time, damage will result.
Also, many controls have registers that remember where components should be. An example would be the ATC arm. If you reset it to a 0 degree position, but the register states that the arm should be at 180 degrees, it can lock up the control, inhibit axis and machine functions, or the control will force the arm to what it thinks is the proper position and crash into anything in its rotational path.
Most of the time, the regulators are a needle valve -- a tapered pin that mates to a similar taper in the fitting. By turning the needle in, the flow will decrease. Some operate the opposite way using a ball and a spring. These regulators often control the exhaust or the release of pressure to the opposite side of the pistons commanded actuation. When the adjustment is complete, the cylinder should work smoothly and timely. Tighten the lock nut (sometimes when locking the nut down the needle will turn slightly) and check the operation again to double-check your work.
Talk to you next month.
As a certified Electronic Engineer, Mike Breen is presently working as a Field Service Engineer with Ellison Machinery Company of Wisconsin. Mike invites you to share your own machine repair and maintenance experiences with him. For comments and inquiries about these articles, his e-mail address is email@example.com.
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|Author:||Breen, Michael W.|
|Publication:||Modern Applications News|
|Date:||Sep 1, 2002|
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