Designing Dowels for Bracket Mounting: How to Determine Size and Load?

[jayanth nivas]

Hi all,
I just started learning engineering mechanics this semester,so what I'm about ask may be a basic question in the subject.I came across a bracket that is used in a industrial equipment (i have attached a sketch as image).The bracket is used to mount a hydraulic cylinder.The cylinder has a clevis which is connected to the eye of the bracket with a pin joint.The bracket is located with two dowels and has 4 bolts connecting it with the the base frame.

My question how to size the dowel?.

My approach is that the two dowels are the ones taking the shear load and the four bolts,with the under head friction prevent the rotation of the bracket.

But the problem if there were no bolts , can the bracket rotate about the dowels ?.Or would it resist the rotation ?

Also what will be the load on the dowel and the bolts ?I'm not sure how to do it.

Thanks for going through the post .And sorry if i have misstated something.

 

[jayanth nivas]

Sorry missed the attachment in last post.

 

[AlephZero]

If there were no bolts, the bracket can slide along the dowels, which is probably not a good design.

The bolts will carry some shear force because of the friction between the bolt head and the nut, and the bracket and base frame. You don't know now much shear is carried by each separate bolt and the dowel, unless you know the "exact" geometrical tolerances when the bracket was assembled.

I would guess the real reason for the dowels is to locate the position of the bracket accurately when the device is assembled, before the bolts are inserted and tightened. The bolt holes must be big enough to let the threads on the bolts pass through. The clearance between the dowels and the holes can be much smaller.

 

[Baluncore]

I believe the dowels are there to position the bracket accurately and to hold a gasket during assembly.
The bolts will have very little shear because tension in the bolts pulls the bracket hard onto it's mounting.
Friction between the plate and it's mount would be increased by the use of a paper gasket.
The manufacturer should have specified a bolt torque and gasket material.

 

[jayanth nivas]

Thanks for the replies

I think i get the point.But given with the task of designing the bolts and dowels how can I go about it ?.I think I'm asking a lot here but I just need the picture or idea to start with.To learn this design I thought i can first size the dowel and bolt and I can check with the actual component (I know that material and bolt class play a role in selection,which I think I can determine).

My approach is if dowel can only prevent the movement of plate in X&Y direction it will be only under shear.Whereas the bolts can prevent the movement in Z direction (perpendicular to this screen),X ,Y and also all the rotations.The applied load primarily tries to move the bracket in X and Y direction ( Taking that load is not parallel to X axis ).Also the applied load is offset to the bracket center.So it will create a turning effect.But what will it turn about ? Can it turn about dowels?.If so will it be 3 kN on two dowels ?. If dowels resist the rotation how can I size the dowels ?.I'm in a bit of confusion pls help me.

 

[Jupiter6]

Baluncore is right on track. The dowels are used for dimensional location and the bolts are used for their tension to make the "joint stick" hence resiting shear. In other words, it's the interface of the parts being mated that resist shear, all because of the compression being put on them by the threaded fasteners (which are in tension).

Threaded fasteners are not supposed to see shear loads. It may work in our garages but it's a big no-no in the design world.

Check out:http://www.roymech.co.uk/Useful_Tables/Screws/Bolted_Joint.html

 

[SteamKing]

Baluncore is right on track. The dowels are used for dimensional location and the bolts are used for their tension to make the "joint stick" hence resiting shear. In other words, it's the interface of the parts being mated that resist shear, all because of the compression being put on them by the threaded fasteners (which are in tension).

Threaded fasteners are not supposed to see shear loads. It may work in our garages but it's a big no-no in the design world.

Check out:http://www.roymech.co.uk/Useful_Tables/Screws/Bolted_Joint.html

To be clear, the threaded portion of the bolt is not supposed to see shear, but bolted connections work not just because of the clamping force put on the parts, but because the plain portion of the bolt shanks undergo shear.

http://www.egr.msu.edu/~harichan/classes/ce405/chap5.pdf

 

[Jupiter6]

To be clear, the threaded portion of the bolt is not supposed to see shear, but bolted connections work not just because of the clamping force put on the parts, but because the plain portion of the bolt shanks undergo shear.

http://www.egr.msu.edu/~harichan/classes/ce405/chap5.pdf

That's a great chapter you quoted there, far better then mine.

I'm not sure what you mean though. I'm assuming you're talking about shoulder bolts which is something that's not involved with the OP's question.

If you're talking about fully-threaded vs. partially threaded cap screws and their allowable shear loads, tell me more.

 

[SteamKing]

No, I'm referring to structural bolts, which have a plain shaft below the bolt head except for the very end, which is threaded enough to accept the matching nut. When a structural assembly is bolted up, the shear loading due to the assembly wanting to slide apart is borne by the plain bolt shaft and not any of the threads. These bolted connections work similar to a riveted connection, with the exception that the bolted connection can be disassembled if needed.

http://www.portlandbolt.com/image/products/full/hex_bolt_drawing.jpg

 

[Jupiter6]

What kind of nut goes on these bolts?

 

[Baluncore]

If the two dowels were important in resisting load they would be far apart.
The bolts are far apart so the bolts are more important in preventing movement.
The bolts should not be in shear, they must be under tension.
The dowel pins will be a tight fit in the mount but a sliding fit in the bracket.

It is important that the plate never moves after the bolts are tightened. If movement occurs, then with every repetitive load / unload cycle, the dowels, bolts or the holes in the plate would work loose and slog out.

When the plate is located on the dowels, the bolt holes in the plate should be sufficiently large that the bolt does not contact the plate except through the washer under the nut or the bolt head. That reduces tolerance requirements in sizing and drilling the holes.

The four bolts lie on a circle, the bracket will try to rotate about the circle centre.
Each bolt creates a friction patch between the plate and the mounting.
The strength of each patch is proportional to friction coefficient * bolt tension.

The offset 3 kN force will force a rotation and a translation of the circle.
Rotation of the plate about the centroid will be prevented by the four friction patches.
Translation of the plate will be prevented by the same four friction patches.

The last thing to do will be to size the dowel pins. The clearance of the dowel pins in the plate will be sufficient to allow for the error in the position of the holes.

 

[Baluncore]

A rivet prevents two plates from moving because it goes in red hot. The rivet was plastic while being hammered so it expands it's diameter to fill the holes in the plates. As it cools it shrinks which pulls the plates together, increasing friction. Unlike bolts in holes with clearance, hot riveting is a self aligning process so there is some ability to handle shear.

 

[SteamKing]

What kind of nut goes on these bolts?

Hex nut as shown in the picture.

 

[SteamKing]

A rivet prevents two plates from moving because it goes in red hot. The rivet was plastic while being hammered so it expands it's diameter to fill the holes in the plates. As it cools it shrinks which pulls the plates together, increasing friction. Unlike bolts in holes with clearance, hot riveting is a self aligning process so there is some ability to handle shear.

Not all rivets are heated before installation. Pop rivets are mechanically deformed during installation without the application of heat. Not all bolt holes will have sloppy clearances with the installed bolt. For some machinery installations, the use of special fitted bolts minimizes the play between the bolt and the hole.

From an engineering standpoint, a structural rivet and a structural bolt are both designed to handle a certain amount of shear force from the pieces of the joint wanting to separate. The strength of the material also determines the number of rivets or bolts required to make an effective joint.

 

[Jupiter6]

To be clear, the threaded portion of the bolt is not supposed to see shear, but bolted connections work not just because of the clamping force put on the parts, but because the plain portion of the bolt shanks undergo shear.

http://www.egr.msu.edu/~harichan/classes/ce405/chap5.pdf

Interesting stuff. Much different than I was taught or practice. Anyways, I think it would be much more productive of you to offer an answer to the OP rather than mess with the guys who give him an answer.