Bolts

Here is site that gives you enough information so you can do the calculations.

metric fastener data

What is the shear strength of grade 8.8 material?

You can use formula-

Cross sectional area of bolt x shear strength of material x Number of bolts you are using

Please note force must be distributed equally on both the bolts.Probably bolt manufacturer can also tell you shear strength of M30 bolt.or you can have a look on machinery handbook.

How big is the lower bolt near the bottom of the frame?

Each of these three bolts has two shear planes to help support the load.

If it were my design, I would make sure that the load can easily be supported with one of those two bolts by itself, as it will be impossible to ensure that the load will always be evenly distributed. Depending on the situation, one bolt could be supporting the entire load momentarily while the other is supporting nothing. But I tend to over design everything I design with large safety factors, especially when the supported load has any potential of causing harm if a component failed. Good luck, be safe, have fun & make it work! Cheers!

When you are in a price competitive situation, you don't dare over design or the $$ go up and you lose the bid!

I would gladly loose the bid if I thought I would need to design an unsafe piece of equipment to get it. I'll let some other fool get the job, kill someone, and go to court. I turn down unsafe work all the time, day and night, Everyone wants it cheep, which is OK as long as you are not compromising safety. A single lawsuit can ruin your career, your business, and your reputation. luckily, I have never experienced one directed at me, but I have seen people loose it all because of a cheaply designed project that fails.

It will make a large difference in the allowable stress whether the threads are in the shear plane or not.

I take it this is a beam runner. If so you need to base your calculations on 3 bolts as you can't guarantee all 4 wheels being in contact.

There is no need to know.

Refer the entire matter to the facility's Engineer/Surveyor for the company providing lifting equipment collapse indemnity insurance cover. Follow the advice given on the periodic inspection reports from this source, which will be in the facility's General Register.

Dont forget to do the same calculations on the plate that you will be hanging the 6 ton from eg leave enough"meat" around the holes to carry the weight

I add a doubt to Zahidlala's: Is possible in his case an extreme shear stress to breake the bolt or a flex effort is what you should expect? Is not distance between bolt bearing something to take in account? or,or? Shouldn't be taken the bolt size directly from a table,i mean empirical data from a machinery handbook? Is really right calculate that in such way?.-

There are many ways to approach solving a bolt problem such as this. The factors you mention would be significant if one wanted to choose the smallest bolt for a 6 ton load...or to determine the largest load the two M30 size bolts could support. However, that is not how the OP phrased the question.

The question posed was could the two M30 bolts support a 6 ton load as depicted in the diagram. I chose to simplify the problem yet in a conservative way such that the answer did not depend on information not provided in the OP. By showing a single bolt could withstand more shear stress than a single bolt loaded with 6 tons would generate, it should be obvious that two bolts would be able to support the same load.

These are 15/16" BSW. The applied vertical load can exceed 10T.