Damper Extension Design

[Ahmad Rezq]

Hello,
I was thinking of the design of an Extension for our direct acting damper. I preferred to go through the problem before checking the other designs.
We have a strong limitation in the machining operations specially Milling.
Post is edited as the Image confuses.
The design is to use a rod with one end with spherical joint to the control arm and the other end with revoulte joint to the damper rod.
The damper body will be revoulte joint to the chassis
the 2 revoultes joints axis are perpendicular

(Mobility analysis)
correct me if i'am wrong.

- Number of bodies to consider : 4 (Damper body, Damper Rod, Extension, Ground)
- Number of Spherical Joints : 1 Extension mounting with the control arm
- Number of Revolute joints : 2 Extension to Damper Rod and Damper Body to Chassis Mount.
- Number of Cylindrical Joints :1 Damper Body to Damper Rod.

DOF= 3X6-3-2X5-4=1

[NickFavazzo]

You should be aiming for 0 DoF
That 1 DoF will allow the assembly to buckle.

Isn't the end of that damper rod threaded anyway? Why not just make an extension that screws on or a longer piston rod (check bucking).

[Ahmad Rezq]

NickFavazzo,

I think that the 1 DOF is The rotation of the Extension+The Damper Rod along the damper axis because of both the cylindrical and the spherical joints.
If I for example replace the spherical joint with another revoulte joint i will prevent 2 DOF so the Mobility of the assembly will be -1 (Over-constrained).

Edit
I was also thinking if I considered only 2 bodies and the ground (Link 1 , Link 2 , Ground)
Revoulte (Link 1 to Link 2 )
Spherical (Link 1 to the ground)
Revoulte (Link 2 to the gorund)
Mobility = 12-3-2X5=-1 (Over constrained).
By adding another body with 6 DOF and Cylindrical joint which cancels 4 DOF then the assembly will have 1 DOF which as i was thinking above.
And the extension and the damper (as one unit) will not have a relative DOF. (Not considering the tolerance of the assembly which will change the picture).

[harshitr2]

Keeping the DOFs aside, don't you think you need a rigid joint where the extensions connects to the damper? As Nick said, why don't just unscrew the damper end and thread in your extension? All you want is to increase the end to end length of the damper, if I understood it correct. This setup wouldn't work, one can tell just by looking at the image!

[Ahmad Rezq]

As Nick said, why don't just unscrew the damper end and thread in your extension
Yes it's simpler solution.


All you want is to increase the end to end length of the damper, if I understood it correct. This setup wouldn't work, one can tell just by looking at the image!

I've some doubts too so that's why I'am asking, If it didn't work then my calculations are missing. and when talking about a rigid joint between the extension and the damper you didn't keep the DOFs aside !

[P^squared]

I really hope Ritwik chimes in soon ( with pictures ) ! Their extended DASD setup looked super clean at comp, couldn't get a closer look though.

I think its just a matter of getting the extended tube as co linear to the piston rod as possible.
That way you will minimise the buckling loads induced near the extension connection.

http://www.fsae.com/forums/showthread.php?12087-Direct-Acting-Spring-Damper/page7
Just in case you missed it :D

[Ahmad Rezq]

I hope that the image doesn't confuse you as the 2 Rev. Axis are perpendicular.
Anyway Using an extension screwed to the damper rod is very good idea.

P^squared.
I've checked the link. and totally agree that any source of compliance will cause a problem.

[harshitr2]

I've some doubts too so that's why I'am asking, If it didn't work then my calculations are missing. and when talking about a rigid joint between the extension and the damper you didn't keep the DOFs aside !

With keeping the DOFs aside, I meant to say that do you really need to do calculations for stuff like this? It's super intuitive, you can tell by looking. I would hope you keep things simple and square, instead of complicating things, and do some calcs for buckling!

[Ahmad Rezq]

I found this Today at Auckland Page.
I was looking for another real pictures from the car but didn't find. I guess it's not complete
https://www.facebook.com/UoAfsae/photos/pb.155875141099200.-2207520000.1454445235./713276522025723/?type=3&theater

[Z]

Ahmad,

Your Kinematic mobility analysis (in your first post) is CORRECT.

But (!), from a Structural point of view I think that design would be BAD. There is no image of the design visible now (you must have deleted it...), but my guess is that it would only work well if the two revolute joints were kinematically very precise, which is to say they would have to be quite long in their axial directions.

A much better solution in practice (IMO) is simply to make a longer version of the standard damper "Eyelet" (ie. the part that screws onto the damper-shaft). From memory, that is what Auckland did for their 2015 car. Their damper extension was a machined aluminium part very similar to the standard eyelet, just longer. For the record, this arrangement has (Edit: Oops... corrected->) DoF = +3, these being axial-sliding of rod in damper body, and rotation of rod (about its axis), AND rotation of damper-body (about its axis), with these rotations being allowed by the lower and upper sphericals. (Hooke's type joints replacing the sphericals would bring DoF back to +1. But this is NOT necessary, because the extra DoFs do no harm.)

Or you might choose any of the other methods mentioned on the DASD thread linked to above.

Z

[Ahmad Rezq]

Z

but my guess is that it would only work well if the two revolute joints were kinematically very precise, which is to say they would have to be quite long in their axial directions.
I was also thinking the same.
IMO theoretically the design would work and the only DOF would be Damper Rod + Extension Rotation about damper axis (Correct me if i'am wrong).
But having the two revoultes very precise IMO is not easy to achieve

[Z]

Ahmad,

Yes. But having only +1 DoF is not necessary for good operation (see also my last post, just edited).

The "two-revolute" approach could be made workable, but it would be a little more bulky. Spherical joints (and "rodends") are so widely used in industry because they make things very easy to design and build. They almost always result in many extra DoFs in the linkage, but that is usually no problem.

Z

[Ahmad Rezq]

Z,
From Kinematic point of view I think any relative degree of freedom between the damper rod and the extension which will change the co-linear relation between both damper and extension axis Must be prevented.

The damper without any extension in direct acting suspension case (2 Spherical mountings and 1 Cylindrical joint between damper body and damper rod) has 2 DOFs. which is IMO better than (-2) DOFs in case of 2 Revoultes mountings + 1 Cylindrical

[Claude Rouelle]

If we look at pure kinematics you do not need any rod end at damper ends: no need of the damper body rotation around its own axis. But once you take the compliance (rocker, chassis) into account....

[Ahmad Rezq]

Mr Claude,
Considering Pure Kinematics. A damper with two revoultes joints has Negative Two (-2) DOFs which means we have to force the damper in its position. While replacing those revolutes with 2 spherical (Rod Ends or whatever) will make the damper have +2 DOFs and we can tell that they are the rotations of both Rod and Damper Body about the damper axis which make the case better.

Another Example is the control arms. From mobility analysis we can tell that if we have an over constrained design or not and this reflects that the control arm will be stressed to fit to its place or not.
IMO These things can be told from Pure Kinematics.

[Claude Rouelle]

I do not even go through the academic count of DOF. I just say that even if you designed, for example, your damper body axis to be in the plane of your rocker, in reality, because of compliance, it won't stay that way.