Bolt Clearance holes 1

[roseda]

In most books I can find, e.g. Machinery's, the recommended clearance holes for bolts increase disproportionally with the diameter of the bolt. I.e. the bigger the bolt, the more diametral clearance it will need.

Can anyone think of a reason why this maybe so.

Why is it not just a constant factor added to the diameter.
E.g. 0.5mm (which would allow for positional tolerances of dia0.35mm on the tapped hole and dia0.15mm on the clearance hole).

The same rule also applies to limits and fits for close tolerance holes.

 

[collinsda]

Hi Roseda,
I would imagine it has something to do with the disproportionate increase in the thread pitch with the diameter of the screw/bolt.
For tightly toleranced holes the hole clearance will increase (to a point) with the diameter of the pin/locating feature for ease of assembly.

 

[meintsi]

roseda -

Think about the perpendicularity issues....
Then you should see why.

Remember...
"If you don't use your head,
your going to have to use your feet."

 

[roseda]

In Reply:

Collinsda:
Thread pitch has nothing to do with the diameter of the
clearance hole.

Meintsi:
Perpendicularity is only an issue when the length of the
clearance hole is taken into account. When I said bigger
bolt, I didn't mean length, I meant bigger diameter.

Machinery's Quote (pp.1536: 26 Edition):
M5 - 5,5 (0.5mm Clearance)
M6 - 6,6 (0.6mm Clearance)
M8 - 9,0 (1.0mm Clearance)
M12 - 13.5 (1.5mm Clearance)

 

[Hush]

Not knowing what was in the minds of the people who set the standards, I'm guessing you need smaller clearances on smaller fasteners because the nut or head is smaller. I.E. if you had a large clearance on a small fastener there wouldn't be enough bearing area for the nut.

 

[meintsi]

The clearance for threaded bolts relates directly to the perpendicularity of the thread pitch. (Especially at the lead-in angles of both the external and internal threads)

This difference multiplies exponetially as factor of the radius from pitchline center. The resultant requirement is a larger allowable tolerance zone for the bolt to be able to be started. (The bigger the bolt, the more room you need to get it started,)

You are not going to put a 8mm bolt thru a 2mm plate, so there is always a thickness to worry about.

Hush is also right with his statement which limits the maximum size of the clearance hole to achieve the desired clamping force.

Remember...
"If you don't use your head,
your going to have to use your feet."

 

[steveh]

One reason for greater clearance on larger bolts is that larger bolts are used on larger assemblies (in many cases).
Large assemblies are often heavy and unwieldy, making it harder to put them together. A little more slop in the clearance holes often goes a long way toward making it easier to put such things together.

 

[diamondjim]

The clearance values are only
the recommended sizes to prevent
excessive bearing loads at the
surface under the washers for these
bolts. Normally the larger
diameter bolts are used on larger
diameter bolt circles and heavier
cross sections. The 5 to one ratio
is kind of a guide for length to
diameter that should be generally
used for dynamic applications.

 

[chili]

I don't know of any formuli or rules so it must be an imperical issue that deals strictly with ease of assembly.

The real question is "why are you concerned about it?" I have always found that if there is a standard, don't deviate from it without rock solid justification. If you deviate, and if you have a failure (too big) or the assembly personnel can't readily assemble it (too small), you're going to be hosed!

 

[svanels]

I go with the opinion not deviate from the standards because they were set from a practical point of view. Ever tried to assemble a head exchanger cover with the use of a bull-pen? Imagine to center a 1.25" diameter bolt. Steven van Els
SAvanEls@cq-link.sr