Designing, Building, and Connecting My Own Wind Turbine
Field Notes from Irricana, Alberta, Canada
by Steven Fahey


Looking for equipment for the tower rigging, I found two types of wire rope hardware:

  • Expensive Crosby U-clamps
  • Inexpensive no-name U-clamps
The difference in price of most parts was a factor of 4-to-1! It all looked similar, but there had to be a difference worth the money. So I bought some of each kind, and set up a test.

For both tests, I used 1/8" aircraft cable, (galvanized steel with 7/19 construction) which has a nominal breaking strength of 1800 to 2000 pounds. In the heavy lifting world, the working load should never be more than 1/4 of the breaking strength. A 1/8" cable should be used for loads that don't exceed 500 pounds.

Two U-clamps were secured to each end of each cable, with a 6" turn-back around a thimble. I attached a come-along and a load cell to each end of the ropes, and use some links and shackles to make connections. I placed a measuring tape on the floor beside the cable, but in the end it wasn't helpful because the photos aren't clear. I took pictures of each test to record the maximum load each cable would resist.


In the picture you can see the un-painted hardware securing the 1/8" wire rope. The load cell measured a maximum of 1000 pounds. Repeatedly during the test, I would crank a higher load, and the cable clamps would slip away. I re-tightened the clamps, cranked again, and they would slip yet again, but with little improvement. When I applied 320 pounds, it slackened off to 290 in about 30 seconds. From 900, it slackened to 800. The thimble distorted at 600 pounds (nothing unusual about that). Eventually the free end of the cable just slid through the first clamp, and that ended the test.

The efficiency of these clips, by supporting 1000 pounds (out of the cable's maximum 2000) is:

1000/2000 = 50%

For a static load of 400 pounds, this cable offers a Safety Factor of:

1000/400 = 2.5

It did not inspire confidence!
The Safe Working Load has been exceeded.
This cable is probably only safe to use for 250 pounds or so.


The load in this test was also applied in steps. As load was applied, this cable also slackened slightly. Never did the cable lose more than 10% of the load. When pulled with 1200 pounds, it only lost 5%. I noticed immediately in this test that the wire rope did not slip through the U-clamps. The wire rope was not moving. This time the slackening must have been due to other deformations, such as the thimbles distorting drastically as the loads got very high in this test. This cable failed at 1670 pounds, when multiple strands broke under a U-clamp's "U". There was some warning that this would happen as several strands broke before reaching 1670 pounds. The tightness of the cable clamps bends the wire rope under the "U", making a stress concentration. This time I was more careful with the measuring tape, and by taking notes, I was able to record the cable's stretch.

These U-clamps are much more efficient:

1670/2000 = 80%

For a static load of 400 pounds, this cable offers a Safety Factor of:

1670/400 = 4.2

The Crosby U-clamps offer enough strength for the working load and the SF.

Lastly, I compared the wire rope manufacturer's elongation data with the stretch measured during my test. They quote a maximum 5% stretch per million pounds. I arrived at 6.5% stretch per million pounds. I can account for the difference by the deformation of the thimbles.

The 1/8" cables made with Crosby hardware will be reliable steadying lines for the gin pole and the tower's mid-section. I expect that the Crosby clamps will perform just as well at 1/4" and 3/8" sizes. Using 1/4" cables for the main lines at the top will give a SWL of 1400 pounds, which is enough for even the worst conditions I expect my tower to experience with an 8 foot diameter windmill. All the jostling and bouncing of lowering and raising the tower makes using 3/8" cable here the natural choice.

Cable Size Cable Strength Crosby Hardware Cheapo Hardware
7 x 19 steel Ultimate SWL Ultimate SWL Ultimate SWL
1/8" 2000 500 1670 417 1000 250
1/4" 7000 1750 5600 1400 3500 875
3/8" 14,400 3600 11,520 2880 7200 1800

All strength figures are given in pounds. "SWL" means "Safe Working Load".
All cables have guaranteed minimum strength values from MIL-W-1511 Standard.
Cables that do not conform to this standard may not have equal strength.
Strength of 1/4" and 3/8" U-clamps extrapolated from 1/8" cable tests.


Both tests:

Segments of 1/8" aircraft cable (galvanized steel with 7/19 construction) were fabricated with two different types of wire-rope clips to demonstrate the cost-benefit of using cheaper clips.

Rated strength of cable = 2000 Lb.
2 clamps secured on each end, 6" turn-back.
Pulled with come-along and 10,000 Lb load cell.
Links and shackles used to make connections same in both tests.
Measuring tape placed on floor to watch stretch (wasn't helpful, in the end).
Photos taken at various load intervals.
Ultimate failure loads were not photographed, because when things break, the come-along goes slack (load goes back to zero).

Princess Auto (China) (80 cents per clip)

Load applied in steps.
When 320 Lb initially applied, some load slackened off to about 290 Lb. (30/320=10%)
Load increased to about 520 Lb. Tension slackened again.
Load increased to about 660 Lb. Thimble distorted.
Load increased to 800 then over 900 pounds. Line slackens again and clips obviously slide.
Load increased quickly to over 1000 pounds, but slackening again back down to <900 pounds.

Failure occurred at the first cable from the left, where the clamp could not maintain enough friction to stop the cable from sliding through.

Crosby Hardware (USA/Canada) (3 dollars per clip)

Load applied in steps. Length measured at 120 Lb = 25-7/16".
Load increased to about 400 Lb. Slackened about 40 Lb (10% load).
Load increased to about 800 Lb. Slackened about 60 Lb (7% load).
Length measured at 760 Lb = 25-9/16".
Load increased to about 1200 Lb. Slackened about 60 Lb (5% load).
Length measured at 1160 Lb = 25-5/8".
Load increased above 1200 Lb until failure at approx 1670 Lb.

Failure occured at 2nd clamp from the right, where the cable broke due to stress concentration at the clamp.

Clip efficiency = 1640/2000 = 80%

Elongation = 3/16" per 1160 pounds = 0.00016"/Lb
Original length = 25-7/16"
Elongation (%) = 0.016"/22-7/16" = 0.0000064%/Lb (6.4%/1,000,000Lb)

Check with Loos & Co.
(download calculator from website): Elongation = 0.0000050%/Lb (5.0%/1,000,000Lb)

Stretch during test higher than theoretical due to thimbles deforming.
Wire rope clips did not slip.