math help

[hammers-n-nails]

Im in the process of figuring out the valve for the traction engine and am trying to figure out how much compression i need if i have about 20.75pounds of reciperating mass(this is an educated guess at this point) and an average piston speed of 3.46fps and a piston area of 15.9 sq.in. how much pressure would i need to counter the pistons movement? im having trouble figuring out how much force the parts have at the end of the stroke, i know all the variables but i cant figure out how to use the equation (i think) i come up with 2.23lbs-force and i dont see how that could possibly be right. the equation i have is F=m(mass? 20lbs)xa(acceleration? 3.46fps) / 34.17(acceleration of gravity)=2.23. if i convert everything to metric and find the newons then convert back to pounds-force i get exactally the same thing. what am i missing? theres no way i only need .14 psi compression. please help im a carpenter not an engineer.

 

[Maryak]

The pressure at the end of the stroke (compression)will be the cylinder volume at BDC/cylinder volume at TDC * 14.7 psi.

Hope this helps.

Best Regards
Bob

 

[hammers-n-nails]

sorry maryak i forgot to menion im talking about a steam engine.

 

[kvom]

The piston does not have constant accelleration as it's speed is sinusoidal.

 

[Jasonb]

Is there actually any compression?

With a double acting cylinder the exhaust is open as the piston travels towards the end of the cyl and then the inlet opens to admit steam to return the piston.

You will get a certain amount of overlap as its usual to have the valves lead by about 20degrees but as the steam is still flowing in any compression will depend on the steam pressure and regulator position.

Jason

 

[Lew_Merrick_PE]

Hammer-n-Nails,

What you are working with is inertia. If the mass is moving in a straight line, then the average acceleration (which may or may not be the value you are looking for) is: a-bar = (vf² - vi²)/2s, where vf is the final speed, vi is the initial speed, and s is the distance traveled. The thing to remember is to keep your units consistent. What you want to find is the maximum speed. Peak acceleration is often a factor of 3-10 times greater than average speed in pyrotechnic events (which you do not have, but that is what I often work with). Rotational inertia is based on the mass of the object and the radius of gyration (squared) based on the shape you are rotating.

I have a paper on rotational dynamics posted at http://www.scribd.com/Lew Merrick that may help.

 

[hammers-n-nails]

http://www.tcsn.net/charlied/

this may be useful