How much force do you REALLY get?

[David Fort]

Thanks everyone for the suggestions and questions. Here is some additional info.

First, as I said, the test was unscientific, and that is a profound understatement. We are pulling a catapult back with a rope. We can pull on that rope while standing on a scale, and see that we pull the rope with about 50 pounds of force.

The direction of pull is not perfectly normal to the scale, maybe 10 degrees off.

There is one pulley in that system to turn the rope up away from the floor so it can be pulled up, pushing our feet down into the scale.

In our catapult system, we have a 2" bore cylinder, and thus think it should have an area of pi.

We are pulling, rather than pushing. The rod has diameter 0.625.
The effective area is the annular area, the cylinder diameter minus the rod cross section area.

Thus, the effective area for retracting is 0.307 square inches.

(all the dimensions in this discussion come from the FIRST Pneumatics Manual. We haven't actually measured them).

The conclusion that we are "getting less than 100 pounds" of force is based on the highly rigorous observation that we can pull the catapult back by hand with 50 pounds of rope tension, as measured by the bathroom scale, and that when we use the 2" cylinder, we can't pull it back. We did not measure the force exerted by the cylinder. Not even with the bathroom scale. (we couldn't figure out how to do that. An idea comes to mind now, but at the time, not).

(the additional 2x is from a 1:2 pulley system which divides the cylinders force in half to get twice the rope travel. Assuming massless frictionless pulleys and all that).

here is a chart of our force/area calculations (which match those listed in the FIRST pneumatics manual):
CylinderDiameter RodDiameter CylinderArea RodArea AnnularArea PushForceLbs RetractForceLbs
0.750 0.250 0.442 0.049 0.393 26.507 23.562
1.500 0.437 1.767 0.150 1.617 106.029 97.030
2.000 0.625 3.142 0.307 2.835 188.496 170.088

There are 3 (fixed position) pulleys in the rope between the cylinder and the catapult to route the rope over the river and through the woods as required to get the whole thing to fit.

Final disclosure: It wasn't a bathroom scale. It was pinched from the wrestling team.

Thanks for your thoughts.

[MrBasse]

Can you post a picture of the set up as it is currently run? The forces in pneumatics are kind of strange as there are a lot of efficiency issues involved. The angle of attack on the cylinder is a major one. You should get more than enough force to pull 50 lbs. We have used a 2" cylinder to lift people before to demonstrate the forces involved.

Thus, the effective area for retracting is 0.307 square inches.

Check your math on that one, don't trust someone else with it every time.


You'll lose some in friction, efficiency, and actual air pressure available. But in no way should you have an issue creating 50 lbs of force.

Check your plumbing, check your air availability, and check the angle you are applying force at. If all of those check out, try a different cylinder as that one isn't doing what it should.

[David Fort]

Originally Posted by MrBasse

Can you post a picture of the set up as it is currently run?

sorry. I didn't think to get a photo while at school. Interesting how frustration hinders thinking.

Originally Posted by MrBasse

Thus, the effective area for retracting is 0.307 square inches.

Check your math on that one, don't trust someone else with it every time.

apologies - I mis-typed. 0.307 is the Rod diameter. the effective area is the piston area minus that,
3.14 sq in - 0.307 sq in = 2.8 sq in.

2.8 sq in * 60 p/sq in = 170 pounds

(I will update my previous post to correct this)

I wonder if this is as stupid as having a bad pressure gauge. It says 60 psi, but this is from the old parts bin and we have had similar disappointments in "Actual Force" on our pneumatics before. Maybe we have a bad gauge lurking. I think that would be a good thing to check, just in case.

[Tem1514 Mentor]

A very simple jig to get some accurate results is to make a 'dead lift' system using those muscle man weights as the load.

Set the cylinder either vertical or horizontal with a pulley.

Just a word of caution. Releasing the load back down will always be very fast unless you are using flow controls. Make sure you are using very strong cording as the tension/shock loads can be very high.

[mentorDon]

Try getting another air pressure gage and place it with a 'T' connector close to the cylinder to verify your air pressure is close to 60 psi.

[David Fort]

Update:
We checked with another pressure gauge - they agree we are working with 60 PSI.

We have straightened out some pulleys and doubled the piston area. Not happy, but seems to be working.

Thanks for the assistance everyone.