Quality Matters

Last week while doing ground work on an antenna I needed a clamp to set a mechanical stop on a ⅜" steel rod. A 5/16" cable clamp is a good fit so that's what I used. I keep a variety of new and lightly used clamps of all types in my junk box for just this kind of occasion.

As I tightened the nuts on the clamp one of them suddenly went slack. I looked down at it and discovered that the saddle had snapped. This was a brand new, never used clamp that still had the label on it. Torque on the nut was very light at no more than ~3 ft-lb since that's all that was necessary for my purpose.

The picture does not show microscopic detail so I'll describe the break. There was no audible sound when it broke. The broken pieces fit together so well that the seam between them is almost invisible. There is no distortion of the steel at the failure surface. The break went right through the ridges in the saddles, so that stress riser was not the problem. The interior metal is clean and crystalline in appearance.

On the basis of this visual evidence I suspect that there was a manufacturing error during the forging process. But that is not at all certain; it's an educated guess. I have never had a failure with this brand of wire rope clamp in the past.

Yet clamps do fail. They fail often enough that just one clamp should never to be used to secure a cable. Two is the minimum and three is considered best practice. Not only must you use this many clamps there is a procedure on clamp placement and tightening sequence in order to achieve the rated holding strength. Of course many people are not aware of the proper procedure or choose not to follow it. In my experience this includes the majority of hams who use cable clamps.

You may be interested to learn what the fine print on that label says, in three languages:

"Do not use where disengagement could cause property damage or serious bodily injury."

A catchall similar to this one is quite common across all industries and products. How well it protects the manufacturer or retailer in case of injury or property damage I cannot say. It is so common that its presence tells us little about the quality of a product, or lack of it.

Like most sensible hams I only use cable clamps for light duty guying and other mechanical applications for tower and antenna work. I always use at least two, even for a brief one time use. Examples include the rigging for lifting tower sections and antenna tram lines. If you must use clamps on EHS guys be sure to use the clamps made for this application, not clamps designed for wire rope and aircraft cable which are more compressible and therefore most tolerant of clamp type.

Of course one should use guy grips for EHS tower guys which are far more reliable. If you shop around you'll also discover that it's cheaper to use a grip than 3 cable clamps designed for EHS. They're also far easier to put on, and to put on properly.

On critical installations never choose false economy over safety. Quality matters. As for that broken clamp? I may take it back to the retailer (a reputable company with many commercial clients) to see what they say. I don't want my money back but they ought to follow up with their supplier.

For another example of a faulty clamp witness the one immediately above. These specialty items have a stainless threaded stud welded to a stainless hose clamp. They are used to secure devices to round masts and yagi booms. This one was supplied with a balun from Balun Designs, which uses two of them to attach the flat balun mounting plate to the boom.

I really don't like breakage like this at the top of a 150' tower. It appears that the welding of the stud weakened the clamp strap. I am left to wonder whether this is a design flaw in which welding heat weakens the stainless steel strap. That would be a pity since they are very useful for antenna work. None of the others in my possession have broken and I replaced the broken one with another from the same company. Time will tell.

As always: be safe out there.