# How do I determine the ‘bolt center’?

Someone asked me today what kind of bolt centers I wanted; he suspects I want 4”. What is he asking and how do I measure it? Is this the length of the bolt?

It’s not the length of the bolt. The technical term for that is “length.”

It would really help to know the context. If it’s automotive wheels, the bolt center is the diameter of the circle described by the centers of the lug bolts or studs. If there’s an even number of these, simply measure from the center of one lug stud or hole to the center of the one directly across from it. If there’s an odd number of these, it’s hard to be exact without special measuring equipment. If you just have to determine whether it’s 4" or 5", that’s easy enough, but it’s pretty tricky to determine whether it’s 4" or 4 1/8".

Well I’m in a factory and it’s for a little bucket thing that bolts onto a conveyor. It’s rectangular and i measured it, and there is no way I can measure that gets me anywhere close to 4" from any side… I think I may just have to call the guy back and admit I have no idea what he wants me to measure. That or just send him a CAD drawing of the bucket.

Surely you are “Putting us on.” [I snuck a peek at your profile!] :rolleyes:

Undoubably you wish to establish a pentagonal configurations of confusion, do you not?
Of course it is not the length but the girth, whatever that is.

OTOH take a 4" dia. bolt, cut through the body with a hand wielded hacksaw, measure the diameter with a micrometer and carefully lay out the centre with a carpenter’s folding rule and center punch the center.

OYAH it may be that the bold centers you so desperately need are countersunk.center drill drilled centers.

Smile, you’re on candid camera.

I figured it out… part of the problem is that we want 4.75" instead of 4". He must have been refering to the center to center distance of the two bolts that hold the bucket on the conveyor.

Thanks for the help though. What part of my profile would lead you to think that

Maybe it’s the part listing your occupation as Mechanical Engineer?

Ah. Now I see it’s the same principle as with the wheels, but a whole lot simpler with only two bolts involved.

All you need is a standard micrometer or caliper. Measure the center-to-center distance of any two adjacent bolts and use the formula r = 1/2( s / (SIN (180 / n))), where r is the radius of the circle defined by the bolt centers, n is the number of bolts and s is the measured distance between adjacent bolt centers.

Yeah, that is why I was reluctant to just tell the guy I didn’t know what he was talking about… it would be like a car salesman saying “4-wheel drive? What’s that mean?”

Now that I figured out what he was talking about, and that his choice of words was not common, I realize it is more like a car salesman saying “What do you mean you want all the wheels to go at once… are you asking for 4-wheel drive?”

Sometimes fighting ignorance is harder than it has to be.

Since you’ve answered that question, what is a “bolt centre”? is it just some measurement that one needs for a task, or is it a physical object?

It’s application for car wheels has been mentioned, but why anyone would want to know the size of the circle the bolts on a car wheel make is beyond me. Size of the bolts yeah, but why the circle?

It’s a measurement – a dimension related to the position or placement of the bolts.

Automotive wheels have several critical dimensions. Among them are diameter, width, offset, and bolt center. If you want to get an aftermarket wheel for your car, it’s necessary for the wheel to have the same number of bolts and the same bolt center as the car itself in order for it to fit.

I would suggest that the problem was in not stating the problem, in it’s entiriety, in clear and concise language.
When a problem is clearly and concisely described the answer can not be far behind.

BTW, without the equation I gave you early, how did you measure the bolt center radius on wheels with an odd number of studs? I’m curious about what sort of tool can do that.

I’m a mechanical engineer, too, and although I knew what a bolt center was from waybackwhen, it wasn’t something I learned in school. I found after graduating that, although I could lay out a four-bar mechanism with aplumb, find the force in a compression spring with flat ends, use Castigliano’s method to find the stress in a curved beam, calculated the entropy and efficacy in a heat exchanger, determine the corriolis force in a spinning wheel, and so forth…I couldn’t spec a hydraulic cylinder or select an appropriate material for a high-temperature, corrosive, minimum weight application to save my life. :o :smack:

I don’t think many of my professors could, either. And I wondered about a lot of my coworkers. In fact, I think I learned more about practical engineering from non-degreed older-than-dirt “designers” and in looking at other peoples’ mistakes than I ever did in school or from “mentors”. And I still don’t know enough to do my job “right”, i.e. without making ignorant mistakes. :mad:

S’okay, though. My current boss doesn’t even know how to change his own oil, so I’m at least one up on him.

Stranger

“Bolt center” is a common measurement in mounted bearings (pillow blocks, flange bearings, etc.) and simply means the distance between the centers of the bolt holes.

I sell industrial components for a living and it is very common for Industrial Engineers to not be familiar with trade jargon. Also, at least a couple of times a year I have to explain to a novice engineer that he can’t dream up his own dimensions for a ball bearing and that he has to plan his design around what is available. You design from catalogs, not from theories.

Sure. There is no way someone can know everything. FlippyFly needs to get over it and just ask. When he was talking to the vendor, he had the perfect opportunity not only to learn, but to make sure he gets exactly what he needs. It’s his job.

There is ignorant, such as “I didn’t know I had to specify a bolt center”, and then there is “I knew I had to specify a bolt center, but didn’t because I didn’t know what it was and didn’t bother to ask”. One is excusable (especially in a new engineer), and the other isn’t.

FlippyFly, no reason to be embarassed. Just ask the guy “what do you mean, bolt center?”. A guy like daffyduck isn’t going to mind. He’d rather sell you the right part the first time.

I am a chemist, and don’t know from bolt centers. I have lived in framed structures all my life, however, and I know that if I am asked for my stud centers, I must reply with the center-to-center distance between the studs. GaryT’s statement that “bolt center” could mean anything else boggled my mind. Briefly.