pneumatic actuated linkage

[rx7rider]

i'm using a pneumatic actuator to run a basic 4 bar linkage. the system works but the motion is quite jerky. what seems to be the problem? any ideas in what to look for in troubleshooting?

 

[budt]

Explain:

"motion is quite jerky"

What type of air actuator are you using? Linear?

Continuous Rotary? Limited Rotary?

Rotary mechanism design type? manufacturer?

Maximum force or torqe required during a cycle?

Force or torque capability of input device?

Consistency level of force throughout a cycle?

Type of Flow Control circuit, Meter In, Meter Out?

Since air is compressible it can be quite dificult to control especialy when there is changing loads during a cycle. This is exagerated when the movement is slow and/or the force changes are abrupt and great.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[rx7rider]

it is a linear actuator from Senior Aerospace, specifically called PrimeMover. i think you can find their specs on their website. when i say jerky, the linkage shakes a bit when the actuator starts up. the linkage works smoothly on its own and so does the actuator. however, when i put it together as a system, i see the erratic movement.

 

[israelkk]

Although Senior Aerospace claims there is no wearing in their design due to the metal bellow, the push pin/rod is still guided inside a bushing. In case you have an even small side load on the pin/rod there will be a greater staring friction than moving friction. Check your design to make sure that the push rod/pin has absolutely no side load when pushing/pulling the 4 bar linkage.

 

[budt]

Since my work is in Industrial and Mobile equipment this info may not be of much use.

I have smoothed starting problems by using a 5-Way valve piped with dual inlets at different regulated pressures to equalize force that can be unequal due to differing areas on the opposite sides of a single rod piston.

Two 3-Ways also work as long as they both shift simultaneously.

Israelkk's mention of the bellows design takes a way piston seal friction but as he says there is still a Rod Bushing and a Seal that must be overcome at start up.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[rx7rider]

Thanks for the insite israelkk. In fact, the actuator does have a side load in the application. It is located on one end of a cantilever type linkage which would naturally give it a side load upon actuation. The linkage itself is around 5 lbs and the length is around 10 in. Do you think that is enough to create the erratic movement?

 

[rx7rider]

One more question: is this phenomenon typical in pneumatic driven actuators? Is this why the use of electrical motorized actuators are more precise and reliable?

 

[israelkk]

No this phenomenon of side load will affect any actuator type. To lower side load effect the push pin/rod has to be guided in two set apart linear ball bearings. The use of ball bearings reduces the friction coefficient to 0.003-0.005 compared to 0.1-0.2 in metal to metal or metal to plastic bearing/bushing. The largest the distance between the bearings, the better. Another possibility is to use a ball joint at the end of the push rod/pin where the ball should be as small as possible.

 

[rx7rider]

Let me see if I understand this correctly and please correct me if I'm wrong. Do you mean that the ball bearings should be designed into the housing of the actuator instead of using the OEM bushing design? In that case, I would totally agree that the starting friction would be minimized. However, I don't think that can be a realistic option for me. I didn't quite grasp the concept of the small ball joint under the push rod. How will that help with the side load?

 

[israelkk]

It was best if the push rod was guided inside the housing of the actuator. The idea with the ball is that theoretically the ball transfers the force only in line with the direction of the push road even if the push force acting on the ball is not in line with the push rod. This is kind of one ball rotational bearing. The problem that the ball "feels" the coefficient friction of 0.1 to 0.2 therefore, the force acting on the ball creates rotational torque on the ball which creates a side load on the push rod. This torque equals the Force X Coefficient of friction X ball radius. Therefore, the smaller the ball the smaller the side load.

 

[rx7rider]

I see. So by putting the small ball in line with the rod axis, it will in fact soak up most of the side moment caused by the weight of the linkage. Is that correct?

 

[israelkk]

Yes

 

[rx7rider]

Thanks for all your help israelkk. I think I have enough information to try to redesign the linkage now.

 

[israelkk]

If you can lubricate the ball and the mating surfaces it will better.