Pressure Exerted on a Coating

No, this is not correct (check the dimensional analysis).

When you torque the two clamps, you create a preload force on each clamp bolt of

P_i = T / (K x D)

where P_i is the preload (lb), T is the torque (in-lb), K is the torque coefficient and D is the bolt diameter (in). However K is dependent on many factors such as thread geometry, lubrication and interface geometry so it will be difficult for you to calculate the actual preload force on the clamps. You might be better off trying to measure the preload (strain gauge, load cell, bolt stretch?).

If you were somehow able to determine P_i, then this is the load (x2 clamps) you would use to determine surface pressure. You would then divide the total load by the actual contact area of the tire patch, which I would assume is less than 15 in² (3x5) due to voids between the treads. This would provide the surface pressure of the tire on the concrete in psi.

We have had conflicting answers. I had assumed that we were applying 80 x (5x3 area) = 1200lbs/sq. in. Am I right ?

No, you are not right.

It is helpful to think these things out a little, to get a sense for reasonableness. 1200 lbs/sq. in means 1200 pounds per square inch, so the force applied by your clamps would need to be 1200 x 15 = 18,000 pounds. Does that seem like a reasonable load for a single tire contact patch? (How much would an "18 wheeler" with such tire loads weigh?)

80 in-lbs is a torque, not a pressure. To find the force exerted by the 2 clamps (if they were arranged correctly), you would look in a clamping force chart for the screw size and thread used. The force exerted would be the sum of the forces from each clamp. Then the pressure would be the total force divided by the area (15 square inches in this case). (This assumes that the steel plate and concrete are quite rigid, so that the force is evenly distributed.)

Lubrication (or lack thereof) of the threads will wreck havoc on your force calculation. If your clamps really look like the drawing, then the screws will be binding in the threads (because of the considerable cantilever), so there is no reliable way to calculate the force, in a meaningful sense: you'd expect there to be lubrication breakdown at various points in the interface between screw and clamp body.

You might want to try applying force with a hydraulic press, in which the fluid pressure times the piston area will give you a reasonably accurate (probably accurate enough for your purposes, if this is just for a rough indication, to compare one formulation with another) indication of force. If you want to be more accurate (for advertising purposes, for instance), then the test rig should be instrumented with a load cell to tell you the actual force applied.

You might shoot for 50 psi, about the highest tire patch loading you will see with cars. For trucks and bicycles the ground loading can be higher, circa 100 psi (i.e the tire patch loading, in PSI, it ends up being essentially equal to tire pressure).

You will want to test with various tire compounds, because some can have a chemical reaction with the coating, and many will vary in adhesiveness. In such tests, the surface to be tested for adhesion is often crosshatched with a sharp knife. A rolling drum test would be more appropriate, because it can better approximate the peeling effect of a tire tread adhering to the paint surface, as it rolls off.

Thanks for your answers. I knew from trying to figure it out that it was not reasonable but still was hoping for an easy solution. The 80 lbs/sq in for each inch simply went over my head. I usually am pretty logical about things. I was thinking about posting some time ago, but tried to figure it out on my own and came up short. Guess I've been missing some of my brain workouts. I have been visiting this site for some time and have enjoyed both of your responses in the past. I am familiar with the crosshatch adhesion test and am knowledgeable of the chemical that is used in the more expensive tires to keep them from breaking down from UV light.

Thanks again for your help. It is much appreciated. Alex

....................for your information, those are just "C" clamps with a nut welded to the top for a torque wrench. And my drawing is completely off. The torque should be over the plate. Not on the side

Don't forget to test this with studded tires.

Hey Tornado,

I used to be a stud. A picture you might recognize from a cruise I took last year .

!!

Yup, I drive by that twice each way every day. Welcome to Rainland!

You must have been on an upper cruise ship deck to get that angle.

I'm just curious where you got those clamps. You should patent them.

Your field must be highly theoretical.

For some empirical data you could contact the state police in your area and find out what some typical axle weights are showing at the weigh station scales in your area, or more quickly ask what the axle weight limits are for a legal load in your state. Measure/estimate the tire contact patch on several eighteen wheelers parked here and there, wherever you can get access. Divide the axle weight by 2, divide that by the number of square inches in the contact patch to get static psi for single wheel sets, adjust as necessary for dual wheel configurations.

"...we are doing a test to determine if a concrete garage coating will delaminate or stain when subjected to the hot tires of a vehicle."

Don't know about the 'stainability-factor', but insofar as the potential for delamination goes, it actually depends a lot (oops; sorry) allot on whether the coating was applied to the concrete in the morning as the sun was rising OR as the sun was setting...

Hmm...how about another test. Go paint a piece of concrete and drive a car over it.

I'm sure you can figure out the rest if you decide to take this route.