Is Your Press Sick? (Critical Press Maintenance – Three Things You Should Know)
Here are some easy ways to check your mechanical presses vital signs:
- Time the flywheel coasting time
- Count the slide strokes using flywheel energy only
- Check your lift clearance with the counterbalance
Flywheel Coasting Time
Flywheels require so little maintenance that they are often overlooked. Most require lube every six months or less. Flywheel bearings have a fairly long life which is typically five years or more. Many shops have no record of the last time they were replaced. But when they fail it can be disastrous. Most flywheel bearings are “special” and may have long lead times. Spun flywheel bearings can cause damage to the flywheel and shaft creating excessive downtime. Or, even worse, flywheel bearings can seize up locking the flywheel to the shaft creating a runaway press that cannot be stopped with the brake.
Before checking the flywheel coasting time check the flywheel brake to determine how it is controlled. Many flywheel brakes are set up to engage when the main motor is turned off. You will need to bypass the flywheel brake to get a meaningful time.
Let the main motor and flywheel get up to full speed. Turn off the main motor and time how long it takes the flywheel to come to a full stop. While timing, watch the flywheel for wobble which can occur at any speed and may change with speed. The flywheel should not wobble at any speed.
The flywheel should coast for over 10 minutes. Every press is different and so are coasting times. I normally get concerned if it is less than 5 minutes and many that I have timed go over 15 minutes. The important thing is to check this on a regular basis and plan for action when you notice a change.
Count the Slide Strokes Using Flywheel Energy Only
This test can indicate lube problems or problems with bronze bushings, crankshafts and slide ways. Some press controls have a “Dead Motor Inch” mode. Use this feature if it is available.
If not do the following: First make sure you are bypassing any flywheel brake. Next bring the main motor up to full speed. Put the press control into the Inch mode. Turn off the main motor and hold the Inch buttons on (you must hold the inch buttons on the entire time). Count the number of strokes the press does before the flywheel stops.
I have seen on new and rebuilt presses that strokes on energy will increase slightly in the first six months of operation. Then the press will go years with basically the same number of strokes on energy. I have seen an accidental overload make significant changes to this number in both directions. A bent crankshaft that reduced this number by 30% and a broken crankshaft that actually increased this number by 30%! Again, every press is different. Check this on a regular basis and plan for action when you notice a change.
Check the Lift Clearance with the Counterbalances
All mechanical presses must have some lift clearance (bearing clearance) to be able to run. And all presses will wear over time. A general rule of thumb is that once these lift clearances double it is time to take action.
A good place to start is to check your manual or contact the press builder (or rebuilder) to determine what the clearances were when the press was assembled at their plant. If you are unable to find the factory clearance numbers start now to establish your baseline.
NOTE: This is a good time to drain any water that has accumulated in these tanks.
Put the slide on bottom dead center and drain the air from the counter balance surge tanks. Put a dial indicator between the slide and bolster in line with each connection with one dial indicator for a single point press, two for a two point press and four for a four point press. Slowly add air to the counterbalance tanks.
NOTE: Caution needs to be used because if the press has problems the slide can go up quickly
Take the counterbalance air pressure up to 80 psi and record the change in the dial indicator readings. This will not tell you all of the lift clearance in the press but it will give you a good indication of the clearance in the slide and connection assemblies. These are typically the first areas to get excessive clearance.
Record these readings on a regular basis. If you see a significant change further investigation needs to be planned. Typically, you are allowed double the original clearance at each clearance point. The clearance at the ball screw may double and require attention when the recorded total lift has only increased 0.010”. Also keep in mind that presses will not heal themselves so if you see less clearance than previously recorded further investigation needs to be planned soon.
I mentioned these measurements should be done on a regular basis. I would recommend every six months for most applications. If a job is known to be particularly hard to run, you may want to do this before and after the run. If an accident or overload occurs these tests should be done before returning the press to production.
Author: Dave Cooper III CHP Systems, Inc.
Edited By: Mike Gruber, BSME Variegate, Inc.
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