Hydraulic cylinder force conversion to torque at knob

[marcusvonbrutus]

I am designing a manually adjustable master cylinder that will adjust the amount of spring compression "preload" for a coil-over-damper setup for our product. This is done by pushing hydraulic fluid from the master cylinder through a hydraulic hose to a hydraulic "ram". The ram will extend or contract through a range to acquire the desired preload.
The range of preload force required is between 90lbs - 300lbs.

Where I need a little help is determining how to calculate the torque at the knob when compressing the spring at predetermined locations.

knowns:

1. force @ ram = force @ cylinder (let's make it 350 lbs)
2. pressure at each component is determined by the area of each component.
3. thread pitch = 1/2"-10 (ACME)
4. radius of knob = 1.5"
5. coefficient of friction (stainless on stainless ACME)= 0.16

I am struggling on how to incorporate the mechanical advantage from the thread pitch into the calculations. I understand that a coarser pitch = harder / finer pitch = easier action at the knob.

My desire is to tweak design parameters to get a reasonable action at the handle.

 

[desertfox]

Hi marcusvonbrutus

You can get a rough estimate of the torque from using the formula's for bolt fastening a rough and ready formula is

T= 0.2*D*F

where F = axial force ie in your case the spring

D = diameter of fastener

0.2 = friction coefficient for unlubricated threads

For a more accurate figure taking thread pitch into account etc you can use this formula from this site:-

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html#Initial_Bolt_Tension

here is one for power screws too:-

http://www.roymech.co.uk/Useful_Tables/Cams_Springs/Power_Screws_1.html

desertfox

 

[marcusvonbrutus]

thanks desertfox.
at a glance - the links look very helpful.
i'll dig into it and and hopefully glean the info i need.