Capscrew size calculation for pneumatic cylinder
Capscrew size calculation for pneumatic cylinder
(OP)
Hi there,
I have been tasked with designing a pneumatic cylinder and was asked to do some calcs on what size bolts I should use to bolt the two endcaps of the cylinder to one another with the honed tube between them (Essentially I need to calculate if 4 off M10 capscrews will be good enough)
Unfortunately I never had the opportunity to complete any tertiary engineering qualification so my theoretical knowledge is abit limited. I am now in a situation where I feel like I am floundering abit and most of the guys at work are too busy to help or are not able to.
Please go easy on me with the explanations and a little patience please.
Some details are as follows:
The cylinder max force will be 9.2kN. There is no cushioning except for an o-ring in the face of the endcaps (The piston bumps into the oring which acts as an endstop bumper). We aim to run the cylinder fast (140 cycles per minute).
I have attached a rough model image to help convey the basic tie rod style design concept a little better.
Thanks
I have been tasked with designing a pneumatic cylinder and was asked to do some calcs on what size bolts I should use to bolt the two endcaps of the cylinder to one another with the honed tube between them (Essentially I need to calculate if 4 off M10 capscrews will be good enough)
Unfortunately I never had the opportunity to complete any tertiary engineering qualification so my theoretical knowledge is abit limited. I am now in a situation where I feel like I am floundering abit and most of the guys at work are too busy to help or are not able to.
Please go easy on me with the explanations and a little patience please.

Some details are as follows:
The cylinder max force will be 9.2kN. There is no cushioning except for an o-ring in the face of the endcaps (The piston bumps into the oring which acts as an endstop bumper). We aim to run the cylinder fast (140 cycles per minute).
I have attached a rough model image to help convey the basic tie rod style design concept a little better.
Thanks





RE: Capscrew size calculation for pneumatic cylinder
Many apparently simple products are not at all simple when studied in the detail necessary to design and build a successful one. Air cylinders are a good example.
Mike Halloran
Pembroke Pines, FL, USA
RE: Capscrew size calculation for pneumatic cylinder
Further the cylinder should be pressure tested to ensure it doesn't fail in service and particularly a pneumatic cylinder as it could fail and go off like a hand grenade.
That said if you want estimate fastener loads,then workout the force from pressure times cylinder bore area and divide by four (for the number of bolts) this will yield the load per bolt.
Divide the force per bolt by the root area of the bolt and obtain the stress, which you can then compare with the allowed bolt stress.
What I have given you is a very rough approximation as a starting point but in reality it is much more complicated as you will have bending in the bolts due to the offset from the resultant force centre.
RE: Capscrew size calculation for pneumatic cylinder
RE: Capscrew size calculation for pneumatic cylinder
Are you are recent grad, or are you not an engineer? Is anyone knowledgable looking over your shoulder? The bolts are not the only thing you need to get right here.
--
JHG
RE: Capscrew size calculation for pneumatic cylinder
There are other engineers in the department and the design is being assessed in design reviews before being manufactured for testing. Originally my design contained 4 off M12 capscrews, however, my engineering manager asked me to consider rather changing them to M10 but said I must do calculations to determine if M10 would be ok. I did not know how to do the calcs so thought I would seek help on this forum as the engineers at the office are often too busy to mentor me.
We would purchase the pneumatic cylinders and have done so prior to my employment, however, they were not up to the task of our machines and did not last so the company started making stronger cylinders themselves (And have been quite successful so far.)
Next week I will need to present the calculations to my manager and tell him if M10 will be ok.
Please guys like I said I am not qualified but I am keen to learn if there is anyone willing to help so any information is appreciated.
RE: Capscrew size calculation for pneumatic cylinder
RE: Capscrew size calculation for pneumatic cylinder
FAQ725-215: Where can I find info on fasteners on the Internet?
FAQ725-536: How do I calculate an assembly torque?
FAQ725-1461: Online Bolt Calculators
RE: Capscrew size calculation for pneumatic cylinder
@ SPDL310 The "bumper" seals on each end of the cylinder are both independent of the pressure chamber seals. I am certainly more concerned about the fatigue life of the bolts but just hope I manage to get enough information together and interpret it in the right way to calculate whether the bolt will hold up.
We have made triplex presses which can run at about 10.2kN and on those they have used M10's successfully but on this press the piston is heavier so I suspect there will be a higher impact load on each cycle. I think the M10's will be ok but my manager likes numbers as do I provided they are right that is.
RE: Capscrew size calculation for pneumatic cylinder
RE: Capscrew size calculation for pneumatic cylinder
If you read the second part of my first post I've outlined the starting point for your calculations.
If your maximum load is 9.2KN then divide this by the four bolts which 2.3KN per bolt, now a grade 10.9 M10 has a proof load of 48.1kN according to this site http://www.roymech.co.uk/Useful_Tables/Screws/Scre....
Now what we don't know is the cylinder material yield strength because if you tighten the cap screw to tight the cylinder material will yield and bolt preload will be lost.
RE: Capscrew size calculation for pneumatic cylinder
RE: Capscrew size calculation for pneumatic cylinder
I finally managed to get some numbers down on paper. Please could you have a look at the attachment and let me know if I have understood your guidance correctly. Could you clarify the following for me please?
Firstly I need to compare the effective force per capscrew to the proof load for the capscrew. (ie. My effective force per capscrew is 2.3kN and the proof load for a grade 12.9 M10 is 56.3kN, therefore this comparison suggests that the M10 is well within its limits for this application.)
I then calculated the effective stress per capscrew to be 45.19MPa. I compared this value to the yield strength of the endcap materials (three to choose from)
Mild Steel - 300 MPa
Aluminum 6082-T6 - 250 MPa
Aluminum 6083-T6 - 214 MPa
All of the materials have a yield strength far higher than the effective stress per capscrew and I would therefore also assume that this further supports indications that M10 capscrews are well within their limits for this application with all the endcap material options.
Are my assumptions above correct and have I interpreted your guidance correctly?
Thanks
RE: Capscrew size calculation for pneumatic cylinder
A.
RE: Capscrew size calculation for pneumatic cylinder
Yes your on the right lines with the calculations you've presented.
Now if the cylinder load is a maximum of 9.2kN then the the combined preload of the four screws needs to be larger than this, so let's say the preload for each screw is 4.6kN, that should ensure that the end cap doesn't move under load.
Now to determine whether the clamped parts yield you need to calculate the area under the screw head which is in contact with the end cap and divide that by the 4.6kN preload, if this exceeds the yield stress for that clamped material then you will need to add more screws so that the preload can be reduced to avoid yielding the clamped components.
Technically you should be following some design code for the design of the cylinder because you might well have to consider bending of the end cap and/or compression of seals/gasket.