How to Calculate the Holding Strength of a steel prob held by a hydraulic rod clamp

[Chris__H]

Hi everyone Merry Christmas to all. I am wondering if some knowledgeable person can kindly help me out with a formula or a calculation to calculate an approximate total holding strength of a steel probing rod being held by a hydraulic cylinder and rod clamps that will have a textured and curved shoe clamps (perhaps Carbide) that will match the diameter of the rod and will cover approximately 80% of the diameter of the surface area. I want to achieve a holding strength of 300 kN. My two unknowns are 1/.the Minimum force of the hydraulic cylinder required, 2/. the minimum height of the shoe pads. I preferably like a smaller shoe height to neatly fit into my design. I have added a cartoon and some pics to hopefully make things more understandable.

also, I believe the frictional coefficient between steel and carbide is 0.4-0.6 (dry surfaces) or 0.1-0.2 (greasy surfaces)(not too sure if this is required in a calculation?)

Thanks for looking,

Kind regards, Chris

 

[hydtools]

300kN = friction coef x normal force
Normal force = cylinder force

Ted

 

[Chris__H]

Thanks for your swift reply Ted really appreciated.
So does this mean: I require a holding force of 600kN, assuming a steel to carbide coef. of 0.5? (60 ton of force seems very high?)

assuming that the clamping area is approx. 134 cm2 so: 134 x 0.5 = 67coef.
Then: the (dry) hydraulic cylinder force required = 300/67 = 600kn

I'm not sure if I'm on the right track here?


Thanks, Chris

 

[desertfox]

Hi Chris H

You don't need the area, just take the 300KN and divide the friction coefficient into it therefore 300/0.5 =



“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[hydtools]

What desertfox said.

Ted

 

[pylfrm]

300kN = friction coef x normal force
Normal force = cylinder force

This is true if the force along the axis of the probe is only reacted by the fixed clamping pad. If the load is shared equally by both pads, then the relevant normal force is twice the cylinder force.

Everything above is based on a conservative assumption that the normal force is parallel to the hydraulic cylinder axis. The curvature of the pads is ignored.


pylfrm

 

[Chris__H]

Thank-you everyone. I'm just wondering if there is a more mechanically efficient way to hydraulically clamp & release my probe rod?

Thanks

 

[hydtools]

How would you define mechanical efficiency?

Ted

 

[BrianE22]

I wouldn't count on getting any help from the pad connected to the rod. As drawn, that pad would have quite a bit more compliance in the rod axis direction than the fixed clamp. I suppose you could add some compliance to the fixed clamp so that it's "rod axis direction" matches that of the piston rod's. Probably not a good idea though since then the actuator bearings would see high side loads.

 

[EngineerTex]

This is a very common application in the oil industry for running pipe. The term is "slip bowl" or "tubing spider." If you want to see an almost identical setup to what you have in your picture, look at a "hydraulic workover rig," also known as a "snubbing unit" on YouTube. Granted, those are much larger than what you have pictured, but note that none of them use the hydraulic force to hold the pipe. The backs of the grippers are held in a traveling assembly against a wedge. The wedge can be either conical or truncated pyramid shaped. In this way, the weight of the pipe, (or when they turn them upside-down and are working against the ejecting force due to pressure in the wellhead) will keep the wedge clamping down on the pipe just like a Chinese finger trap. This way, the harder the force against it, the greater the gripping action of the inserts.

To get you going a little further along the way, note that the wedge is a 6:1 angle. A shallower angle does not grip the member very well while a steeper angle has a tendency to deform the slip bowl to the point that it will get wedged in and not let go of the pipe.

Using a wedge also may factor into the overall safety of your system. On the rigs that use this type of slip bowl assembly, the slip bowl is capable of holding the entire string weight, even with total hydraulic/pneumatic power loss.

EDIT:
What I had intended to add to this before I hit the "Submit Post" button is that if you design your hydraulic system so that the clamping cylinder has 6 times the cross-sectional area of the lifting/snubbing cylinders, then you can tie the clamping cylinder to the lifting/snubbing cylinders and you should always have enough force to hold the member being held. If it were me, I would do it this way and add some items to the overall hydraulic system, including check valves, sequence valves and pilot-operated check valves to make sure that if power is lost, the weight of the member being held will also keep pressure on the gripping cylinder.

Engineering is not the science behind building. It is the science behind not building.

 

[BrianE22]

A way to get both pads to contribute to the friction is to set up the pads like what you have with a disc brake on a car. The pads float and each applies force to the rod.

 

[Chris__H]

Thanks again, Ted, Tex & Brian. Somehow the commercial probing machines are able to apply at least 200kN to lock the soil probe in a tiny compact unit whilst not appearing to be using massive amounts of force as we have been discussing. I have attached a few Youtube examples. I also spoke to a company in Germany called sitema, that do spring and hydraulic rod locking mechanisms (pic attached) they are also at a loss to how they are doing it? Any thoughts? Maybe it just smoke and mirrors? ha ha.

Here are similar YouTube videos of the soil probing process with the time frames between 1:28 – 1:44 : Link and also here (however this one is manual): Link

 

[BrianE22]

The Sitema devices are using a split, tapered ring to generate the large clamping force. The spring moves the ring axially and the tapering causes the splint ring to shrink in the radial direction. Hydraulics push the ring upwards (against the spring) so the split ring ID grows and releases the rod.

There's a lot of radial pressure so your probe would need to be able to handle that pressure.

 

[SwinnyGG]

The Sitema site has a diagram of how their devices work.

It's literally a small-scale, hydraulically or pneumatically released version of what ETex described as used for drill string applications.

 

[EngineerTex]

What caused Sitema to be at a loss? It seems that their clamping system (or something close enough) is exactly what both of the videos are showing. Why do they think that these systems in the videos are not like what they have?

From my knowledge of the systems I have seen (that are NOT for ground probes):

1) The parts that contact the central member had teeth and were not just holding based on friction.
2) Higher trip speed was accomplished by simply sliding the teeth against the central member during the retract stroke. This may be what is already going on in the videos, but I don't think that taking time to open the jaws will significantly reduce trip speed.
3) Sliding the teeth against the probe will significantly reduce the life of the teeth. I presume that something similar would happen by wearing down the friction surface if the system relies on friction to hold the probe.
4) If you are using teeth as opposed to a friction surface, then you are dealing with something that is not described by a friction equation.

Engineering is not the science behind building. It is the science behind not building.

 

[Chris__H]

Hi EngineerTex, Thanks so much for your comments. Apparently, Sitema has quite tight clearance tolerances and when this is combined with outdoor use and dirt/frequent cleaning, they feel their products were not suitable. Also, the HRC for the passing rod itself in a Sitema locking system has to have a HRC >56 whilst standard pushing rods have a HRC 36-40 (These push rods have 10mm walls). So you might be right about teeth. Attached is another probe machine that has a similar setup to what I originally envisaged regarding the clamping system for pushing and pulling the rods. This one has a capacity to push & Pull 200kN. Judging by the size of the hydraulic cylinder that is being used, I don't believe the friction equation is being used here? Therefore, it must be an equation with some type of tooth setup? I thought they'd simply be a toothed & curved clamping jaw (pic also attached). If this is the case? Do you or anyone reading have an idea or/& equation that I could possibly use to get a similar result of 200kN push and pull holding capacity, Please?
Also a big Thanks to Brian & jgKRI for their comments.

 

[EngineerTex]

I think that any equation could only be extremely sensitive to very small changes in regards to any sharpness of tooth and relative hardnesses between the teeth and the probe. Any wear on either part will change what they are capable of holding.

This is a project that would require real-life testing. As a first guess, I would think that you would want to maximize the hydraulic cylinder area and simply keep a pressure reducing valve on the cylinder and reduce the pressure to a fairly low point for a first guess. Next, run the probe down and pull it back up and see if it slips. If it does, turn up the pressure and continue in this way until you have it reliably clamped. On the other end of the spectrum, you could apply the maximum possible pressure and determine if that damages the probe then reduce pressure for your working pressure until it doesn't damage the probe.

Engineering is not the science behind building. It is the science behind not building.