How to find: X" bolt holds "Y" lbs weight?

[JStieger]

I'm not a mechanical engineer by far so a lot of projects go like this "well this bolt looks strong enough!"



How do you really know? Charts and tables anywhere?



For example, I want to build a bracket to hang a Racor filter off my frame. I am thinking 10 lbs total weight so how many bolts of what size and thread pitch will give the holding power I need without major overkill?



-Thanks!-

 

[rbattelle]

I'm not a mechanical engineer by far so a lot of projects go like this "well this bolt looks strong enough!"



How do you really know? Charts and tables anywhere?



For example, I want to build a bracket to hang a Racor filter off my frame. I am thinking 10 lbs total weight so how many bolts of what size and thread pitch will give the holding power I need without major overkill?



-Thanks!-

I have 2 words for you: Machinery's Handbook.



In regard to your filter, if it were me I'd fabricate a bracket that uses four 1/4-20 stainless steel hex bolts, nuts, washers, and split locks. But if the mounting were particularly precarious I'd use 3/8-16 stainless hex bolts. All told, most of the stuff I build goes together with one of those two sizes.



The "correct" way to determine the fastener, however, is to determine the tensile stress area of the bolt, then calculate the maximum load. Then you can calculate the required torque to apply to the bolt or screw based on the margin you need with your ultimate tensile stress. All the necessary formulae and tables/charts/data are contained in that wonderful little tool called "Machinery's Handbook".



Usually, I just overdesign everything.



-Ryan

 

[nickleinonen]

The "correct" way to determine the fastener, however, is to determine the tensile stress area of the bolt, then calculate the maximum load.

or you can just overbuild it... my reverse lights are bolted to brackets with 3/8" capscrews, and the brackets are bolted to the frame ends with 1/2" bolts. the whole light/bracket combo only weighs like 2 lbs... [1lb light/1lb bracket]

 

[rbattelle]

or you can just overbuild it...

Amen to that!



-Ryan

 

[JStieger]

or you can just overbuild it...

Amen to that!



-Ryan

Well that is what I usually do (I hope)!



But where I am going with this is that DC says not to weld on the 3rd gen hydroformed frames. This is not an issue to me because I can't weld worth beans anyway (which could also be a bad thing I guess :-laf ). So the alternative is to drill and tap into the frame for a mounting bracket. These frames are thin walled so I would want to use a fine thread fastener, but I don't want to drill any hole larger than it needs to be. Basically I want to move my Holley Blue to right by the tank on the inner frame rail and then mount the Racor on the outside frame rail - probably off a bracket of the bed support ala the FASS method.

 

[Bill Lins]

Just use one of them Ford bed bolts- according to their commercials, just one of them can lift the whole truck! ;-)

 

[nickleinonen]

Well that is what I usually do (I hope)!



But where I am going with this is that DC says not to weld on the 3rd gen hydroformed frames. This is not an issue to me because I can't weld worth beans anyway (which could also be a bad thing I guess :-laf ). So the alternative is to drill and tap into the frame for a mounting bracket. These frames are thin walled so I would want to use a fine thread fastener, but I don't want to drill any hole larger than it needs to be. Basically I want to move my Holley Blue to right by the tank on the inner frame rail and then mount the Racor on the outside frame rail - probably off a bracket of the bed support ala the FASS method.

how about using them nut inserts. . drill your hole, install insert and use tool to colapse insert. instant threads in metal...



inserts & tool

 

[JMichel]

http://www.futek.com/boltcalc.asp

 

[rbattelle]

So the alternative is to drill and tap into the frame for a mounting bracket. These frames are thin walled so I would want to use a fine thread fastener, but I don't want to drill any hole larger than it needs to be.

I had the same concern, so rather than drill/tap the frame I fabricated a bracket that bolts to the rear transmission crossmember instead. The holes in the member are through-holes (not tapped) into which I inserted 1/4-20 bolts/nuts. No problems, and my fuel pump is HEAVY (maybe 8 lb or so). You could easily drill holes on both sides of the crossmember and run your fuel lines across it so that you've got a pump on one side and the Racor on the other.



Such a mounting is convenient because: 1) it offers a fail safe; if one of the bolts fails, the unit just drops down onto the cross member instead of directly onto the highway, and 2) the crossmember acts as a "skid plate" for your stuff.



Why not mount your Racor in the engine compartment? That's where I mounted my filter... pics in this thread.



I can snap a picture of my fuel pump mount if it would help...



-Ryan

 

[JStieger]

Why not mount your Racor in the engine compartment? That's where I mounted my filter... pics in this thread.



I can snap a picture of my fuel pump mount if it would help...



-Ryan

Ryan-



I would really appreciate it if you could snap a few pictures both inside your engine compartment at different angles as well as the fuel pump mount.



Thanks!

 

[rbattelle]

Ryan-



I would really appreciate it if you could snap a few pictures both inside your engine compartment at different angles as well as the fuel pump mount.



Thanks!

No problemo, but it'll have to wait until tomorrow. Me tired, need sleep.



-Ryan

 

[JStieger]

http://www.futek.com/boltcalc.asp

Thanks! I just bookmarked this site.

 

[cojhl2]

Two ways to look at this.



Start underdesign, and continue to increase until it does not break. That way you have defined the loading.



The other way is to overdesign in the first place then it will never break.



The prob is: its hard to know what the load is going to be... Remember Impact Loading, shake, vibration, whatever you wanna callit. . Its hard to define without simulation.



Also ya gotta take into consideration place to which you are bolting. There is always a weakest link somewhere.

 

[jwgary]

Just because the bolt can hold the load doesn't mean the hole can. You are talking about mounting something to the side of the thin frame rail? That puts the fasteners in shear instead of tensile load which lesens their capacity by about half. The thin wall that you are threading into will not support the load for very long. Usually you want 2X the thread diameter of engaged length on the thread. I would be more comfortable with drilling all the way through the frame rail and using a nut and washer on the back side. JMHO as a mechanical engineer.

 

[JStieger]

You are talking about mounting something to the side of the thin frame rail? That puts the fasteners in shear instead of tensile load which lesens their capacity by about half. The thin wall that you are threading into will not support the load for very long.

I've been thinking about this too for the last couple weeks and agree with you. I'm going to fab up a bracket instead that will bolt to a bed support - similar to the FASS system. The bracket will then have the RACOR and Holley blue bolted to it. This morning I am going to go out with some cardboard to make some templates.



I decided to go the bracket route after I found I can't make the RACOR fit under the hood without it starting to look like a Powerstroke or Duramax - i. e. , a cramped dog's breakfast!