shear capacity of dowels 2

[rattler]

Is there any data published on shear capacity of plain dowel pins eg BS 1804 or EN ISO 8734? These standards give hardness values but no indication of shear strengths or load capacities. I have estimated a shear capacity using a formula for hardness vs tensile strength but am not sure how accurate this is?

 

[jbel]

Someone out there correct me if I am wrong but dowel pins are not designed to carry a load. They are strictly used for locating purposes. The load should be carried by a something else in an assembly, such as bolts or studs. While a dowel pin may be able to physically carry a load there is no reason to publish this information because it is contrary to the design intent of the part.

 

[rattler]

I am using the dowels to locate two faces which are bolted together. In order to locate, the dowel hole clearance has to be less than the bolt clearance - I think the dowels therefore have to be capable of carrying the shear load on the joint. The result is that the bolts won't "see" any shear load.

 

[jbel]

rattler,

I think what is going in here is that the bolts, when torqued to the proper spec, will see the load before the dowels ever would. If the two parts were assembled without any bolts and only dowels then the dowel would see load, but the bolts are preventing the assembly from moving and therefore carrying any side load.

 

[CoryPad]

General design practice is for dowels to carry shear forces while bolts/screws carry tension forces. That is why dowels are tight tolerance and bolts/screws are loose tolerance. As for shear capacity, here is the general formula:

&[ignore]tau[/ignore]; = F/A

where

&[ignore]tau[/ignore]; = shear stress acting on dowel
F = applied force
A = dowel cross-sectional area

If dowel yielding is the allowable failure mode, then:

&[ignore]tau[/ignore]; < &[ignore]tau[/ignore];_Y

where

&[ignore]tau[/ignore];_Y = dowel material shear yield strength

&[ignore]tau[/ignore];_Y = 0.577 &[ignore]sigma[/ignore];_Y

where

&[ignore]sigma[/ignore];_Y = dowel material tensile yield strength

If dowel fracture is the allowable failure mode, then:

&[ignore]tau[/ignore]; < &[ignore]tau[/ignore];_U

where

&[ignore]tau[/ignore];_U = dowel material ultimate shear strength

&[ignore]tau[/ignore];_U ~ 0.6 &[ignore]sigma[/ignore];_U

where

&[ignore]sigma[/ignore];_U = dowel material ultimate tensile strength

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

&quot;General design practice is for dowels to carry shear forces while bolts/screws carry tension forces. &quot;

But surely your design typically carries the shear as friction between the two surfaces generated via bolt tension, rather than direct shear in the dowels?

Cheers

Greg Locock

 

[unclesyd]

We use quite a few gear pumps that are fabricated from D-2 steel that require very close fitting dowels. We use (3)x1&quot; dia. hardened dowels on the pump to set the internal clearances 0.0005&quot; on center lines of the shafts. The dowels are not designed to carry any of the shear load. But in checking the design it was found out that the dowel pins could carry any shear load that could be imposed by the operation of the pump. These pins locate (4) 2&quot; dia. holes 4.5&quot; apart.
The tighter the tolerance on the dowels the harder it makes any interchangeability of the pump parts. The interchange of the pump housing components requires that the dowels be enlarged and the shaft bores be bused and reground.

We have a number of smaller gear pumps that have 2 dowels and they are designed to carry a shear load imposed by very high pressure differential across the gears. As these dowels are not being used for location the tolerances of the dowel pins are looser. We have almost complete interchange of parts on these pumps.

It really depends on what you need the pins to do. The best way is to determine what you need strength wise and work backwards. Keep with 2 pins if possible. Watch pins near the edge. Watch the pin interference also if near a an edge or hole.

 

[CoryPad]

Greg,

Good question. I was trying to speak from the most general perspective. I define a dowel pin as an interference fit cylinder that is used to transfer shear forces. Most bolted joints do not contain these, but those that do use the dowel to resist shear forces and screws (if any) to resist tension forces. Bolted joints usually have three varieties: tension, shear-bearing, and shear-friction. Most joints are shear-friction - screws are used with oversized holes and shear force resistance is provided by the friction force developed by the fastener preload. Shear-bearing has the fastener in a close fit hole so that the fastener takes shear forces by the hole edge bearing against the screw shank. Lastly, tension joints have screws in oversized holes with essentially no shear forces, so the only forces are tension/compression.

Perhaps others refer to locating pins as dowels. These pins are loose fit and provide no force resistance. Then, the joint is one of the three above, usually shear-friction.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[chicopee]

If you have to calculate shear strength of a dowel pin multiply its cross sectional area by shear stress of material. Typical values for shear stress are:
10,000 psi for Al.
25,000 psi for Cu.
35,000 psi for bronze
35,000 psi for hot rolled .1% steel
45,000 psi for cold rolled .1% steel
60,000 psi for Stainless steel
65,000 psi for silicon steel

 

[rattler]

Thanks for everyone's reply. I have designed the joint in the way CoryPad describes however I don't have the strength properties for the material. The standard EN ISO 8734 only gives a hardness value and a composition. I have used a formula to convert hardness to strength for a material (Su = 3.45 HB), but I don't know how accurate this is?
Cheers

 

[CoryPad]

rattler,

Assuming you have standard material composition and processing, your formula ought to be correct within ~10%. If you need more accuracy, you can perform simple testing to obtain the shear strength of the pin.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jcrowell]

The SPS Unbrako catalog gives the &quot;Calculated double shear strength&quot; of its dowel pins.

 

[WCL]

Be sure the material the dowel is pressed into and reacts against across the joint is strong enough to carry those shear forces. We use a lot of gray iron and nodular iron castings. Our dowels are for location only (centerlines for shafts and bearings). To carry shear load into the mating material, sometimes the dowel needs to be larger than what its strength capabilities are to make sure the mating material isn't crushed.

 

[MintJulep]

My version of the SPS catalog lists single shear strength. I can't immediately see a way to use a dowel effectively in a double shear application.

The formula that SPS cites is shear strength (based on conversion from hardness) x pi x D^2 / 4

 

[rattler]

I can't find anything on dowels on the SPS website. Could you please post a link to the shear strengths?

Cheers
Rattler