Linear Actuators

[paulengr 44]

I have an existing system which has a huge hydraulic unit to power up essentially two devices. The first is a turntable to reverse every other item on the production line. It's an ancient (1950's vintage) unit with a chain driven hydraulic motor. This is easy. Just replace the motor with a drive and a gear motor. The second is slightly more complicated. It's a 36" linear actuator. It needs to move at a speed of about 3-4 inches per second minimum to duplicate what the old hydraulic cylinder did. I don't have an exact force requirement but I'd guess 100-200 lbs. is sufficient. The existing hydraulic one will actually overdo it and pinch the product and jam up the whole mechanism if the ultrasonic proximity switch fails to detect the product. I went looking...but the problem I noticed is that with electrical linear actuators it seems that at least the ball screw versions come in either 6-24" lengths or they are very slow (0.5") but with a lot of power (tons of pressure). Any sources which are on the "light" (as in speedy but low thrust) but long end of things?

[TimWilborne 108]

Getting the screw lead you need shouldn't be a problem, but most of the brands come standard with a maximum stroke of 1 meter. Don't know if that is close enough to 36" or not. Many of them will do custom lengths at a premium. I would take a close look at how sturdy the guides are on your machine. A linear actuator will not hold up to side loads and misalignment like a hydraulic cylinder will.

[Ken Moore 23]

Would an air cylinder work? With a 2" cylinder and 80psig air, you would have the force required, and the speed is not an issue.

[ianbuckley 0]

You will need to get a better estimate for force requirements, because these might not have enough, but they are plenty fast and long enough: www.linmot.com. Our testing has shown them to be very responsive.

[paulengr 44]

I can get better force estimates but my first thought was to look at the 800 pound gorilla in the room (Danaher). Then when I started looking around, it seemed like for instance IAI was selling to the "short" and sometimes "low power" end of things. Others were selling actuators that could lift buildings a few inches in a single hour...the opposite situation. So far the only one I found that seems to be hitting this kind of stroke length and power range is Nook. E-Force sells very nice units too with the appropriate stroke but they're designed for much higher force outputs.

[Peter Nachtwey 15]

hydraulic systems and designs have come a long way in 50 years and no doubt your hydraulic system has seen better years. Hydraulic pumps are about 95% efficient. You should be able to get by with a much smaller pump IF you use an accumulator to store the energy during the idle or dwell times. One small pump should be able to supply the needs of both devices. Hydraulics have an advantage in those cases where the duty cycle is low and there is a need for moving masses at high speeds. A proper design would use one pump to supply the average needs of both devices. During idle and dwell time the excess energy is stored in the accumulator. The accumulator supplies the make up oil during peak demands. The advantage is that one pump can be sized to supply the average needs of two devices instead of have two motors where each must be sized to meet the peak loads. Electric motors have a big advantage when the duty cycle is 100% like on a conveyor. This is at the edge between the linear motors and hydraulic systems. Have you looked at Exlar. Our customers have had pretty good luck with those. Edit, air cylinders would work as long as one doesn't need to accelerate or decelerate quickly. Air cylinders are much harder to control that hydraulic cylinders or linear motors. Air is not significantly cheaper than hydraulics in fact it could be more expensive because the control is more complicated. paulengr should look at linear motors first and if he can't find a linear motor solution then I know how he can upgrade is hydraulic system. I just wanted to point out that there are tradeoffs that are made between hydraulic and linear motor systems. From a controls standpoint it makes no difference. Both the linear motors or hydraulics can be controlled using the same controller. Edited 9 Feb 2008 by Peter Nachtwey

[paulengr 44]

Have not looked at Exlar. Mechanical maintenance is much different from electrical in the plant. Their standards are essentially that they don't have any. The guy who is over maintenance keeps trying to tell me that all hydraulic systems leak....all of the hydraulic systems would turn off even a herd of hogs in this plant. I have worked in a marine (mining no less) environment so I know he is full of you know what...if your hydraulic system is leaking oil out, it's also sucking dirt in! That being said, almost all of the hydraulic systems are equipped with compensator systems so that the pump can be mechanically "idled". This makes it at least somewhat more efficient. Since cycle times are nowhere near consistent at the current time, your strategy of using an accumulator probably wouldn't work compared to a compensator...unless I simply modulated the pump electrically (on/off). We are working on consistency however. The way that the system evolved is that over the past 10-20 years, they moved from a purely manual system to one where now an operator pushes a button to excite each machine cycle. The major project now is to automate the entire line as a unit, so that the entire system runs at a consistent (and maximum) production rate as if it is one large machine...similar to a strip mill, paper mill, etc.

[Keith Menges 0]

I don't think Peter is advocating using an accumulator instead of a variable displacement pump. I think he is advocating using an accumulator in addition to a variable displacement pump. In that case you could use a smaller pump and motor and allow the pump to refill the accumulator during the dwell time. This would have the dual benefit of providing better response as the pump compensator doesn't neefd to try and react instantaneously to an increase in flow and the idle energy loss will be lower with the smaller pump/motor combination. Keith

[Peter Nachtwey 15]

Yes, but simple fixed displacement on-off pumps will work but better yet is a VFD driving a fixed displacement pump. Instead of the pump responding to the drop in pressure, the pump is activated by the flow demands as calculated by the motion controller. The pressure feedback is still used as a trim but does not supply the main control signal to the VFD. The extra cost of the VFD is offset by the cheaper pump. Meanwhile the pump is turned at the rate the motion controller determines. The whole system works together. I am not a fan of pressure compensated pumps because. 1. They are slow. They react instead of anticipate. 2. They have a fixed proportional band and this is plain wrong and I don't see why the pump OEMs can't see it. I can expand on that but... 3 difficult if not impossible to model for critical applications. This goes back to my "it isn't designed if it doesn't have a transfer function stamped on it". Check out the linear motors first. Parker and Moog have just released a new version of their linear motors. I feel confident you can find a linear motor for the low speeds and masses you are talking about. If you can't find a linear motor to do the job then I have some hydraulic suggestions. Hydraulic design and techniques have come a long way in the last few years.