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Maintenance Free Bolting

Maintenance Free Bolting

Maintenance Free Bolting


Does anyone have any experience with using maintenance free bolting solutions for either wind turbine foundation or tower bolts? An example would be the IHF Stretch System as detailed at the following link:


I have a few questions about this:

1. I think the overall idea is that these bolts will be hydraulically stretched prior to tightening the nut. This eliminates friction losses and torsion stresses in the bolt resulting in a primarily axial stress being applied to the anchor bolt. Thus, greater axial post tensioning forces can be achieved in a purely axially loaded bolt than one loaded with torsion stresses and frictional losses. This makes sense to me and seems like a great benefit. However, how does this eliminate bolt maintenance as this tensioning system would not eliminate relaxation, concrete, creep, etc.?
2. How would the IHF Stretching System compare to other hydraulic stretching methods?
3. Is it common for tower bolts to be hydraulically tensioned? Or are they torqued? In the past, I have seen that it is common for foundation anchor bolts to be hydraulically stretched, but I haven't watched tower bolts being tensioned.
4. What other competing maintenance free bolting options are available?

Any feedback or opinions would be appreciated, even if you are not familiar with the IHF Stretching System. Ultimately, we are looking for a system that eliminates performing tensioning maintenance for our wind turbine bolts.


RE: Maintenance Free Bolting

1. yes, that describes the method, and we have used it on the (very large, very high tension) nuts clamping turbine casing together successfully. It is MUCH more reliable at several things than ANY other tensioning method: Tension is "set" by the hydraulic motion, NOT the pull of the nut on the threads. Preset tension is consistent between all nolts in the set. Tension is set accurately at each stud/bolt at each stage of the tensioning process (30%, 70%, 90%, and then final.) No galling damage when relieved, little or change because of nut or thread damage or slight dirt or differences in lubrication on each thread.

It is more expensive, but you get a MUCH more consistent, higher reliability joint. I have not used these on wind turbine towers, but believe the change should be considered if you need a "better bolt tension" for the next design. It is faster assembly too.
2. Can't tell. See the different sales people. Try not to use heat-tension/heat-stretch systems, since you are outdoors in a dirty construction site under widely diffrent weather temperatures and wind.
3. Don't know.
4. "Maintenance free" is not really true - You'd not want to be disassembly these things regularly though. Easier assembly? yes.

RE: Maintenance Free Bolting


I'm late to the party but i found this information about tensioning on the Tentec website.

The calculation appears to try and take into account relaxation, we use tensioning devices for machine frame installation and I believe once there in we never touch them again although I think we reccomend a check on the tension periodically.

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

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