B.S. 951 Bonding Clamp.

Perhaps it helps with thermal movement as the pipe expands and contracts with temperature.  I think the clamp bands and clamp bodies may have different expansion coefficients?  Generally good practice though.

Actually the BS951 clamp is not a particularly good design, there were better clamp designs with springs that do not suffer from the thermal loosening, but they are not BSI approved. It is quite possible to fit a BS951 clamp in a way that the screw is tight, but the contact band isn't and it can be very hard to tell by visual inspection (though it becomes painfully clear after a good pull) In that sense a worm drive hose clip would be better..

It is very telling that most of the DNOs have their own clamping arrangements for pipes and clamp jackets, that are properly sprung and eschew the traditional '951, presumably for this reason.

I think the lock nut is just a sop to cover  the poor performance.

In equipment I have designed I do not like them, and try to use almost anything else when contact to pipe or armour is required.

Constant force springs in various sizes feature quite a bit.

i always thought the lock nut was tightened sufficiently so that it would not be possible to unscrew the bolt with just a screwdriver, it would require two tools.  Mechanically reinforcing the "do not remove" message. It backfires a bit since if you only have a screwdriver, you just disconnect the terminations leaving the wires dangling, as seen around a meter in a recent thread. It should be able to be lockable at both connections, or at least a different screw head on the terminal screw.


Edit; just found BS951 online, the lock nut is there solely to maintain the tightness of the screwed connection, it gives maximum torque values for various sizes of screws, which the lock nut maintains. Nothing to do with two tools, oh well.

I can't see that pipe expansion or contraction will create any rotational force on the main fixing screw so the locking nut is pointless. On fixing the clamp, the band will crush the pipe slightly thus creating a tight permanent grip for the bonding clamp.


Z.

Oh yes that F******* label, for something so flimsy it can deal out some damage ? however I visited a house, back in the days of the 15th Edition where we would bond the garden gate, that the owner, I quote, " an accomplished handyman" had installed the new kitchen but needed a socket for the washing machine checked. The socket as is always the case was buried at the back of the cupboard under the sink so getting to it was fun, however, the fun stopped when i felt a burning pain on my arm, this turned out to be a fairly impressive cut caused by the strap on the bonding clamp that said "accomplished Handyman" had fitted and then cut off the extra tail leaving basically a bacon slicer for the unwary plumber/electrician. A few words of guidance were given and said handyman somewhat sheepishly made the tea and we had a laugh about it, eventually!!

In reference to the OPs question, I had always assumed it was a simple lock nut purely to prevent the screw from loosening off but also handy for securing the label in position.

I can not agree that the main fixing screw has any rotational forces acting upon it sufficient to loosen it when correctly installed. Friction will keep it well and truly in place. Just consider wheel bolts or nuts on a car wheel. The bolts on my vehicle have to tolerate great temperature changes and vibration. They do not come loose, even without locking nuts.


Z.

Wheel nuts are very carefully designed with a cone shaped area of contact between the wheel and the nut or bolt head, so that there is plenty of friction at the largest possible radius, and the design is self centring so that the circle of contact is more or less guaranteed to be complete. A normal nut is flat, or may be slightly high in the centre and that means that there need not be a large area of contact. On a car with studs and nuts, if you put the nuts on backwards, so the conical surfaces are facing out, or use ordinary nuts of the same thread, instead of the mating ones, it soon comes undone, and chews the thread on the studs as a bonus,


More importantly to the BS951 clamp, the thing the nut is pressing against is bent up from flat stock, and is not re-finished, so the surface quality is not guaranteed, and the tolerances on the threading are such the bolt may be anything between quite tight or finger loose. They are made to a price, and as such the tolerancing is not great.

Just consider wheel bolts or nuts on a car wheel. The bolts on my vehicle have to tolerate great temperature changes and vibration. They do not come loose, even without locking nuts.

Wheel nuts have quite a reputation for coming loose - that's why these days they're multiply redundant (there are many on each wheel so one or even two can fail without risking the wheel coming off), torqued very carefully when altered and on larger vehicles have movement indicators added (those little plastic arrows).

   - Andy.

So, who has seen bonding main clamp screws coming loose of their own devices? I do not think that I have.


Z.