Torsion Bar to support Tow Bar
Torsion Bar to support Tow Bar
(OP)
I'm new to the site and so far have found alot of good info. I am working on designing a torsion bar to hold a tow bar of a trailer up. The tow bar pivots about two pins and I need a torsion bar set up to hold the tow bar in a level position when not hooked up. What I'm needing is a little help calculating the size of my torsion bar. I have it modeled as a hex shaft. The coupling is 7" ahead of the pivot (could be moved if need be) and the weight of the tow bar is 505 lbs. at 52" from the pivot. I have two torsion bars modeled up held at the ends and free in the middle coming together to one link. The bars are 12" long each. Could somebody help me with material type and how to determine if I have a large enough bar. To me it seems small but my calculations say I have enough. Oh yes I need about 20 degrees range of motion, 10 up and 10 down from level. I would like it level and then an additional pulling force could be applied to gain the 10 degrees up travel. Any help would be greatly appreciated. Thanks in advance.
Also I've uploaded a couple of PDF's to see what I'm working on. Might be more clear.
Also I've uploaded a couple of PDF's to see what I'm working on. Might be more clear.





RE: Torsion Bar to support Tow Bar
There is only one pdf not two, however if I understand correctly the hexagon shaft terminates with two bannana shape links onto a rod with eyes, so is the torsion bar pushing upwards?
If so, the ratio of forces from the pivot point ie 52" * 505lb divided by the distance from the above pivot point to the distance to the vertical rod with eyes and that should get you your vertical force on the bannana shaped link.
That force then multiplied by the distance to your torsion bar will give you the torsion on the spring.
desertfox
RE: Torsion Bar to support Tow Bar
RE: Torsion Bar to support Tow Bar
Well the standard torsional equation will be a good starting point.
I assume from your earlier post that the torsion bar is clamped centrally against rotation and the rotation take place at each end.
So the formula you need is the standard one ie:-
τ= T*r/J
where τ= shear stress
r = radius of shaft
J= polar second moment of area
T= torque on shaft
also you will need:- for the stiffness and deflection:-
T/θ = G*J/l
where G= modulus of rigidity
θ = angular defl in radians
l= length of shaft
other symbols as before.
Now the one difficulty you have is the hexagon shaft because it J value is not as easy to find as a circular section however you could approximate it by assuming a circular section of a diameter equal to the across flats dimension.
desertfox
RE: Torsion Bar to support Tow Bar
Thanks again desertfox for the response. I remember this stuff but its been a long while since messing with spring stuff :)
RE: Torsion Bar to support Tow Bar
Not sure how good your torsion bar will be if you pivot in the middle usually one end is fixed and the other turns, you could do with considering a torsion spring or two springs. basically both halfs of your shaft want to twist in the same direction at the centre which isen't ideal.
I'll have a nose in some of my books.
desertfox
RE: Torsion Bar to support Tow Bar
RE: Torsion Bar to support Tow Bar
Ah I see you have two seperate springs okay that sounds better, so now you use the formla I provided earlier, you have your load from your earlier calculation and you know your deflection +/_ 10 degrees.
You will need to deflect each spring a little it first to give it some pre-load then your final deflection needs to meet your desired load.
You need to check the shear stress you calculate with the allowable shear stress for the material your using, a rough rule of thumb is the max shear stress is about 50% of tensile strength.
good luck
desertfox
desertfox
RE: Torsion Bar to support Tow Bar
RE: Torsion Bar to support Tow Bar
Thinking further I am wondering about the movement of the tow bar doesn't it move through 90 degrees? if thats the case the torsion bar will have to do the same movement so your +/_ 10 degrees is out of the window.
desertfox
RE: Torsion Bar to support Tow Bar
RE: Torsion Bar to support Tow Bar
Ah I thought it might be put vertical when not in use how you doing with the calcs, if you upload them I'll have a look at them.
desertfox
RE: Torsion Bar to support Tow Bar
RE: Torsion Bar to support Tow Bar
Thanks be interesting to see, if the shaft is 30" long whats it deflection like?
desertfox
RE: Torsion Bar to support Tow Bar
RE: Torsion Bar to support Tow Bar
I used your torque from the last post, assumed a 1" dia shaft and I agree with your stress but deflection I get to be 18.57 degrees but I assume it isn't that critical.
desertfox
RE: Torsion Bar to support Tow Bar