Bolted Joint Design: Impact Of Adding Spacers To Increase Fastener Length 1

[mecheia]

All,

I've been working on a bolted joint design, and am interested in some thoughts and feedback in general.

I have attached a picture of two scenarios both being used in the same application. The design consists of an iron bracket or flange clamped against a large iron base. The iron base has a threaded hole in it. A stud and nut is used to secure the bracket or flange to the iron base. The two scenarios are: (1) a short stud with a nut directly on the surface of the bracket, or (2) a long stud with a steel spacer between the nut and the bracket to increase fastener length.

Load conditions could be: (1) loads applied to the bracket which lets assume for the purpose of the discussion act directly in line with the fastener, or (2) thermal growth due to high joint temperatures.

In general I know longer spacers are advantageous due to the increased stretch for the same preload making them more tolerant to initial losses in clamp load, and a higher joint stiffness to fastener stiffness ratio will reduce the percentage of external load the fastener will absorb.

John Bickford's book on bolted joint design does discuss the use of spacers or washers as a way to change the stiffness ratio between the fastener and the joint in order to reduce the percentage of a load change that will be absorbed by the fastener; however, he doesn't go into any calculations.

If I consider the steel spacer as part of the joint and do my stiffness calculations for the fastener and the joint, while the fastener stiffness does go down due to the increased length, the joint stiffness also reduces at a faster rate. It seems to impact the joint to fastener stiffness ratio negatively, and the fastener would tend to absorb more load due to an external load.

The spacer doesn't seem to provide any advantage from a stiffness ratio standpoint.

Any thoughts?

MechEIa

 

[Tmoose]

What happens if you increase the outside diameter of the spacer 2X ?

What is the nature of your loads ( millions of full load cycles, build it once and bump carelessly it with a forktruck a few times a month, ??? ).
What's your maximum service load?
How often will the temperature cycle?

 

[mecheia]

For this particular application I am limited on space, I could make it slightly larger but not two times the diameter. It will see millions of cycles mainly due to temperature of the components involved. With that said it isn't a critical joint from a safety standpoint.

In general I am not too concerned about this particular application, but more in general how to approach this analytically and from a joint best practice standpoint:
- John Bickford's book often discusses calculating stiffness for the bolt, nut, and washer system and how it should be considered in the joint design.
- That leads me to think instead of including the spacer in my joint stiffness component, I should consider it as part of the fastener stiffness.
- As I have it considered currently in the joint portion of the stiffness calculation it would help to make it 2x larger as eventually it reaches a point where it does help reduce the impact of external load on the fastener, but typically I am limited on space.
- If I consider the spacer as part of the fastener stiffness it could help and may indicate the separate spacer component would be advantageous; however, if I consider it as part of the joint I would probably be better off making it part of the bracket by extending up the casting and going a little larger in diameter.

Really mainly trying to understand the physics involved, and from a joint best practice standpoint if the spacer makes sense.

Thank you.

 

[Tmoose]

It used to be when riding up the chair lift the lift tower etc anchor bolts would have long spacers above the base plate.

http://thumbs.dreamstime.com/z/ski-lift-station-blue-sky-23932233.jpg

I always figured that was to get into the range of 7X fastener diameters as recommended in some MIL spex

Nowadays not nearly so much.

http://www.washingtonpost.com/blogs/worldviews/files/2013/12/Hkg9318603.jpg

I think properly done non-epoxy anchors in concrete automatically include significant grip length in the concrete anyhow, although with the danger of corrosion attach without heroic measures, especially outdoors or below grade.

http://1.bp.blogspot.com/-4DnBYUiEUMY/Td5j6MVKX_I/AAAAAAAABEc/x2xxkv0v5g0/s1600/Image+001.png

If your loads aren't too high some combination of belleville washers may be a good way to go.