Force from a piston

[kiwi2]

Hi All,

Can anyone tell me whether the force from the piston is affected by the size of the piston rod?
For example if you have a 1/2" diameter piston (area .196 square inches) and a 1/4" piston rod (area .049 square inches) is the effective piston area only .196 - .049 = .147 square inches when the pressure is applied to the piston rod side of the piston?
I guess what I want to know is whether the piston rod should be kept as thin as possible.

Thanks,
Alan C.

 

[Jasonb]

I suppose there does come a time when the piston rod takes up more room in the cylinder and it starts to affect performance. Don't know if the same amount of steam is addmitted through the valve on each stroke then the pressure will be slightly higher in the rod side due to the lower volume so that may cancel things out a bit.

Length of piston rod would also come into the equasion, if the engine has a long stroke or distance from cylinder to crosshead is long then a larger dia rod would be needed

There probably is a formula somewhere for the proportion of rod dia to piston dia for a given working pressure.

1/4" would be quite large on a 1/2" piston, 1/8" would be more usual

 

[Blogwitch]

Kiwi2,
You are quite correct in your assumptiom, and is the reason liquid spring shock absorbers work.
Even though both top and bottom surfaces are open to each other, because of the difference in area of the two sides, the liquid spring will always extend, thus forcing the ram out. It is the same method used in the airstruts that are used on modern day cars and some of our machinery.
Just by taking the surface area of each side of the piston, and multiplying it by the pressure inside the cylinder will easily give you the force present on each side of the piston, and if calculated out, can give you whatever force is required to lift or support the weight.
If you require equal force from both sides, then you use a piston rod that goes through the piston, and having a gland at each end of the cylinder where the piston rod protrudes through both ends. In this case, both sides are closed to each other. This is a method used a lot on steam engines or hydraulically operated machines where the same force is required for both fwds/bkwds stroke.

Clear as mud, but I hope it gives you a little insight.

John

 

[Charles Lamont]

In a double acting steam engine the piston rod is typically about 1/6 the diameter of the piston, so you would loose 1/36 of the area. Valve setting generally takes no account of this, so the out stroke produces a bit more power than the in stroke.

 

[kiwi2]

Thanks all.

I thought that the area on the piston occupied by the rod would affect the force but I wasn't sure.
On a 1/2" piston a 1/4" rod has an area which is 25% of the piston, 3/16" is 14% and 1/8" is 6%.
I think 3/16" is starting to get into the negligible range.
The comment about air struts was interesting - I wasn't aware they worked like that.

Regards,
Alan C.

 

[Philipintexas]

I built a double-acting, six-cycle, IC engine and after achieving adequate sealing of the rings and valves, used a pressure gauge on each spark-plug hole to "balance" the location of the piston. During construction I left adequate threading so I could move the piston either way in the cylinder to get nearly the same pressure in either end. Volume may not be the same but equal pressure made for a smooth running engine.