Hydraulic Actuator Limits

[khonfahm]

Hi All,

Thank you for taking the time to read this post.

I am inquiring to see if these sort of speeds are achievable with a hydraulic actuator.

80 mm travel. Time : 8ms AVG Speed: 10m/s . Opening force: 1100N
70 mm travel. AVG Speed: 8.75 m/s Time: 8m/s. Opening force: 1100N
60 mm travel. etc...
50mm travel. etc...

Basically a device that can vary distances between 2-80mm, but open in 8 milliseconds, and seperate a 1100N force.

I'm guessing with this sort of speed/acceleration requirements some sort of braking device will be required too(?). I was reading on a different thread something about "hydraulic piston cushion"

Before I begin researching this topic further, I just thought it might be worthwhile to know if these accelerations and speeds are achievable with a hydraulic actuator? Also if it is, how big do you think the pump and the reservoirs will have to be?


Thank you for your time.

 

[HPost]

There are plenty of cylinders that can achieve that velocity. They have low friction seals that can cope with friction. The compromise however is the oil loss. The seal selection depends on the load holding you want need. In general, low friction seals don't hold loads very well. For high velocity, load holing actuators, with good position control and low response time, you are looking at servo cylinders as used on simulators. They can get quite expensive, so you need to define the application in order to get the correct actuator for the right price.

HPost CEng MIMechE

 

[PNachtwey]

I thought I had responded to this post.
Just like usual the hydraulic guys start put with the components without looking into what they must do.
A rough calculation assuming that 1/3 of the time ramping up, 1/3 of the time at constant velocity and the last 1/3 ramping down results in a peak speed of 15 m/s and acceleration rates of about 852g. The 852g is a killer. If khonfahm expects to do this with only 1100N of force then it will be tough to accelerate the piston and rod let alone any load.

852g!!!! I could lower this by a little by assuming 1/2 of the time is accelerating and the other half is decelerating but it wouldn't make any difference really and the peak speed would increase to 20 m/s





Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[khonfahm]

Thanks for the input. As Pnachtwey pointed out - the velocity is not the issue here. It is the Acceleration.


I need to be able to open and close the switch in 1/2 a 60hz cycle (8.333m/s). If the contact seperation distance is 80 mm.. this means 10 m/s average speed.

HPost - When you say they can get quite expensive - what sort of pricing are you talking about (this would be good to know as a ball park figure)? $10k+, $100k+ $200k+ USD?

 

[PNachtwey]

Khonfahm, Moog bought a company called California Linear Devices. The CLD linear actuator isn't hydraulic. Basically it is a linear actuator that is similar to a voice coil in a speaker. These are the fastest linear actuators I have seen yet they are not fast enough for your application.

http://www.moog.com/products/motors-servomotors/linear-motors/

Khonfahm, what you haven't told us is how much mass you really need to move. This would tell us if some sort of gearing and be used.




Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[HPost]

The details I gave only considered velocity. It was assumed that there would be over travel at each end to fulfill the necessary acceleration and deceleration.

It would be impossible to achieve the acc and dec with a fluid system if the entire actuator length is contained within the stated stroke.

The cost would be towards the lower end because of the small size.

HPost CEng MIMechE

 

[LittleInch]

I understand you're searching around, but please don't double post. If you think it's in the wrong place, then red flag your own post or at the very least add a link to the other post (here for anyone intersted

http://www.eng-tips.com/viewthread.cfm?qid=352225

).



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[EdDanzer]

It may be possible to do this if more information about the application is known.

What is an acceptable acceleration and deceleration time and distance? Is it acceptable to have a higher force available during acceleration? Is positioning accuracy required? How frequently will it be actuating? Does it have to accurate both directions at that velocity? What temperature will it be operating in? Are there any other parameters that effect design like fire resistance, safety codes, size, and available power to drive the system?

I have designed a pneumatic system that will reach this type of speed and have proposed a hydraulic version and looked at electric versions. This application is an acoustic signal generator, what is your application?

Cost could range from $2000.00 to $50,000.00 depending on your application or it may not be physically possible.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[PNachtwey]

I get asked these problems all the time. I have a Mathcad worksheet that uses about 10 differential equations that will provide answers in no time. Over the phone I use my rule of thumb that assumes 1/3 of the time is used for ramp up, ramp down and constant velocity. The equations are MaxVelocity=1.5*MoveDistance/MoveTime and AverageAcceleration=4.5*MoveDistance/MoveTime^2. The peak acceleration is 1.5 times the AverageAcceleration. I can bang out these calculations on a calculator. More often that not these simple equations are all I need as in this case. These equations are gold because I can screen out the obvious without resorting to the Mathcad work sheets. The Mathcad work sheet takes into account cycle times, accumulators and required flow.

Khonfham should have provided the mass to move as well as any external force that needs to be overcome. Providing the force is useless. The NET force is determined by the peak acceleration * mass + frictional+gravitational forces.

When I get a question like this my first thoughts are watch out, this guy needs help and probably more than he is willing to pay for.

@Ed, you must have customers that are willing to shell out the big bucks. I can see reaching these speeds with pneumatics on a single shot basis. There are some videos of shooting ping pong balls at super sonic speeds on YouTube. I can see this being repeated often.






Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com