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.