Push or pull

[tree grower]

General question about whether there is greater power if a two-way hydraulic cylinder pushes or pulls, and how the position of a pivot point factors in. As an example, the dipper on an excavator is extended by a cylinder that pulls, while the dipper on a backhoe uses a cylinder that pushes. My particular interest relates to design and construction of a light-duty grapple loader , but the push vs pull question is the same.

 

[JCB]

You might get a bigger force on the push, simply because the connecting rod isn't using up some of the surface area for the fluid pressure to work against.

 

[AKKAMAAN]

General question about whether there is greater power if a two-way hydraulic cylinder pushes or pulls, and how the position of a pivot point factors in. As an example, the dipper on an excavator is extended by a cylinder that pulls, while the dipper on a backhoe uses a cylinder that pushes. My particular interest relates to design and construction of a light-duty grapple loader , but the push vs pull question is the same.

Push creates more FORCE than pull. Pull creates more SPEED than push..

example
a 4" bore diameter cylinder with a 2" diameter rod will have 12.57 inch^2 area on capped side, and 12.57 - 3.14 = 9.43 inch^2 area on the rod side
cylinder will push 12570 lbs and pull 9430lbs, for each 1000psi pressure

speeds will be about 3"/sec pushing, and 4"/sec pulling, for each 10 GPM pump flow.....

 

[RickB]

Push creates more FORCE than pull. Pull creates more SPEED than push..

example
a 4" bore diameter cylinder with a 2" diameter rod will have 12.57 inch^2 area on capped side, and 12.57 - 3.14 = 9.43 inch^2 area on the rod side
cylinder will push 12570 lbs and pull 9430lbs, for each 1000psi pressure

speeds will be about 3"/sec pushing, and 4"/sec pulling, for each 10 GPM pump flow.....

Nice work, and a great example.

 

[J_J]

Using a hyd cylinder to push against an object which is perpendicular to the push angle, will push at the cyl rated force. Any other angle will produce less force, because of the geometry. A 4 in cyl with a 2 in rod, and 3000 psi, at 90 degrees to the object will push 37,699 lbs of force, and pull 28,274 lbs.

Now change the angle to 45 degrees to the object being pushed, and the force decreases, because using the same cyl, the push force will be 28,647 lbs, and the pull 19,993 lbs.

If you look at any of the lift cyl on a tractor, they are lifting at an angle, and not straight on. Those cylinders are capable of pushing much more, if connected at 90 degrees to the lift arms.

The distance of the cyl lifting point, to the end lifting point will affect how much the machine can lift. The longer the arm , the less weight can be lifted.

 

[kennyd]

There are some great calculators here that you can plug in real numbers as you design your setup: Baum Hydraulics Corp :: Spec Calculator

 

[Brad_Blazer]

To get force at the tip of a working arm multiply cylinder force by the minimum (perpindicular) distance from the cylinder's line of action to the arm pivot point. Then divide by the distance from the tip to the pivot point.

The available force will change continuously with position. I made a spreadsheet using the law of cosines to graph force and speed over the range of motion for various geometry choices.

 

[AKKAMAAN]

Nice work, and a great example.

Thanks RickB!!:drink:

 

[tree grower]

Thanks, guys. This thread will go in my design folder

 

[Iplayfarmer]

Thanks, guys. This thread will go in my design folder

This is not what you are asking, but I believe it belongs in the same discussion. The attachment of the cylinder relative to the pivot or articulatio joint and the load makes a difference in the forces on the structure of the machine. If a cylinder is pushing (or pulling) on a point between the articulation joint and the load, the stresses on the articulation joint will be less. If the articulation joint is between the load and the cylinder attachment, the stresses on the joint are more.

Your example of a excavator vs a backhoe is a good illustration. The backhoe cylinder that is pushing the dipper to extend is putting less force on the articulation joint because it is taking some of that force. The articulation joint on the excavator has to handle not only the force of the resistance of the load, but also the added force of the cylinder retracting.

In all of these scenarios, the effect doesn't change at the end of the bucket or grapple or whatever you're building. The difference is seen in the articulation joints on the machine.

 

[AKKAMAAN]

Trying to follow the excavator part of the thread .....the so called dipper arm is the one that hold the bucket...right!?....and the dipper arm is jointed to the boom....
I just cant find the difference between backhoe and excavator, in how dipper cylinder is attached.....:confused2:

There is a difference on the boom cylinder placement...

Who wants a backhoe with the dipper cylinder under the boom???

I'm just not following this part....Please explain....

 

[Iplayfarmer]

Trying to follow the excavator part of the thread .....the so called dipper arm is the one that hold the bucket...right!?....and the dipper arm is jointed to the boom....
I just cant find the difference between backhoe and excavator, in how dipper cylinder is attached.....:confused2:

There is a difference on the boom cylinder placement...

Who wants a backhoe with the dipper cylinder under the boom???

I'm just not following this part....Please explain....

In the two pics you posted, the dipper is run the same... cylinder pulls to extend backhoe. I haven't seen many with the dipper cylinder on the underside of the boom, but I assume that's what the OP was referring to. In the case of the pics you posted, the booms are what are different. The excavator boom is raised by the extension stroke of the cylinders. The backhoe boom is raised by the retraction stroke.

 

[picker77]

Push creates more FORCE than pull. Pull creates more SPEED than push..

example
a 4" bore diameter cylinder with a 2" diameter rod will have 12.57 inch^2 area on capped side, and 12.57 - 3.14 = 9.43 inch^2 area on the rod side
cylinder will push 12570 lbs and pull 9430lbs, for each 1000psi pressure

speeds will be about 3"/sec pushing, and 4"/sec pulling, for each 10 GPM pump flow.....

In accordance with this concept, a hydraulic top link should logically be installed with the rod end toward the tractor, so the end with the most piston area (the non-rod end) is the end that will resist that heavy load--right?

I ask because I'm getting ready to install HTL on my tractor later this week. I can hang the cylinder in either direction, but having the rod end toward the implement would probably be easiest for hose connections and routing, and generate the least amount of hose movement during use. However, I've seen HTL installations done both ways. So is there really a right and wrong way, or is the difference so small it's best to just do it so it's most convenient?

 

[radman1]

In accordance with this concept, a hydraulic top link should logically be installed with the rod end toward the tractor, so the end with the most piston area (the non-rod end) is the end that will resist that heavy load--right?

I ask because I'm getting ready to install HTL on my tractor later this week. I can hang the cylinder in either direction, but having the rod end toward the implement would probably be easiest for hose connections and routing, and generate the least amount of hose movement during use. However, I've seen HTL installations done both ways. So is there really a right and wrong way, or is the difference so small it's best to just do it so it's most convenient?

As far as the cylinder is concerned, it does not matter which end is front or back. The forces are the same on the cylinder. However, I recommend you install with the rod/stroke facing toward the back and away from the tractor. Why? If the rod faces away from the tractor (toward the implement) the hoses will not be changing length as you retract and extend the cylinder. Otherwise with an 8" stroke on the top link, the hoses will also have to extend and retract as the rest of the cylinder moves. It is hard enough to get the postion of the hoses correct when raising and lowering an implement and not have to deal also with the change in the hydraulic extension/retraction.
I live in farm country and lots of HTLs on ag tractors. To be honest, I have never seen a HTL attached with the rod/stroke end attached to the tractor. It is not the best method.

 

[Iplayfarmer]

I have seen pictures of backhoe attachments that have the rod end toward the tractor. On these, the hoses are routed to the middle of the boom and double back to attach to the cylinder.

As far as force is concerned it's not going to matter. If your design lends itself better to routing hoses for a rod end near the tractor, do it that way.