A tale of two states and an ACME screw axis

An ACME screw story

In our last posting we talked about the advantages and disadvantages of ball screws and ACME (lead) screws. In the next two postings I’ll share two stories, one about the Road Runner and the other about Wile E Coyote…If you’re just joining us and this makes little sense to you, you better back up at least five postings to catch up and see what this is all about. Actually, one story is about an ACME screw and the other is about a ball screw.

Meet the vertical axis

The ACME screw story is about a multi-axis system that has one vertical axis. We built the system and tested it in Texas, then shipped to a manufacturer in New Hampshire in the month of February. The pre-shipping testing in Texas went well, but the same test run in New Hampshire failed after about 15 minutes.  The failure mode was that the servo system running the vertical axis would over current and “trip out” to protect the motor and amplifier from overheating. If you reset and homed all the axes, which took a few minutes and started the test again, it would run for about fifteen minutes and fail in the same way. Everything about the system was the same in Texas as it was in New Hampshire except for one thing. Do you want to guess what might happen before you read further? There was no shipping damage either, so don’t go there. Wow, could this be “Puzzler^©™^®” for “Car Talk^©™^®?”

In Texas, they ran the test move profiles in a lab environment at about seventy-five degrees.

From Texas to in New Hampshire

The factory floor in New Hampshire was about forty degrees. Now the owners weren’t trying to save money on heating costs, it was just the nature of the business. It was a woodworking shop and there were exhaust hoods over the wood cutting areas and over the paint/varnish finishing areas.  Lots of cold fresh air was coming in to keep the dust and fumes away from the workers and the warm air would go out through the ventilation hoods. The building’s heaters could only get the factory floor up to forty degrees. It was close to zero degrees Fahrenheit outside that day. The ambient temperature at the system was the only thing different between the test run In Texas and the test run in New Hampshire.

Ambient temperature the only difference

So what’s going on? We monitored the servo motor’s current via a computer connection to its amplifier. The current was low when the test began and it would build up as we got closer to the fifteen minute mark and the amplifier would final trip. Reset and home all the axes and we would see the same pattern again. The duty cycle of the vertical axis in this colder environment caused the Teflon nut/ACME screw to heat up and expand to where the friction level between the two got so high that the servo motor didn’t have enough torque to move the load without overheating the motor. We ran the test again and just before the current got to the trip point we’d lubricate the ACME screw. This did two things, one it cooled the screw down and of course lubricated the assembly. We’d see the current drop back to normal as soon as they applied the lubricant. Fifteen minutes later, we had to do the same thing all over again. So the solution was to have someone stand there and spray lubricant every fifteen minutes…

Lower duty cycle saved the day

Did you really think that was the solution? Nooo, the real solution was we didn’t have to do anything. That was because the test’s duty cycle was much faster than the actual wood working duty cycle where the wood product would be cut. All the axes went through the same motions, but at a much slower speed when they were actually doing work. Thus, the vertical axis would never move as often or as fast in the real wood working application as it did in the test cycle. Then we had coffee and donuts while we watched it run flawlessly making the wood product that it was designed for without lubricating the screw every fifteen minutes.

The advantage of the ACME screw in this story is that it didn’t back drive and drop the vertical axis when the amplifier tripped out. The disadvantage was that it couldn’t handle the test’s high duty cycle in the colder environment.

The ball screw story will be the next time.