The law of unexpected consequences

Published: Friday, 06 October 2017

THERE appears to be a new reason causing the closure of locks—namely the failure of balance beams. Ignore the timber thefts from Walsall Locks, writes Jimmy Lockwood.

colwick struggle2For some time there have been reports of gates being out of balance and either closing when they should be open or, even more annoying, opening when they should be closed. There are also some that are very stiff to move.

Balance beams

So now let’s think about our school days, probably before we were interested in canals, when many of us were taught mechanics and particularly balances, turning moments, forces, etc. Now apply those thoughts to balance beams. A gate moving from its preferred position of rest requires a force to be applied to the balance beam greater than the force exerted by the gate at rest. A person applies the force and hey presto the gate moves to the desired position.

[The lock gates shown above are those of Colwick Lock on the Trent & Mersey Canal, even when replaced as can be seen are way out of balance and a struggle to move, The lock gates below are those of the dreaded Aston Broad Lock also on the Trent & Mersey that have been unbalanced for many, many years and extremely difficult to move—Editor.]

aston lock 2However if the gate is out of balance the weight of the gate is now the greater force and the gate returns to its preferred position quickly and stops suddenly. The mass of the balance beam travels at an increasing speed in an arc from rest until the sudden stop. However, the mass would like to continue to travel and so there is potential for a damaging conflict of forces which will be exercised at the point where the beam joins the gate.

Weaken the timber

Okay, so if it only happened once it is unlikely a problem will be registered. Repeated occurrence over a long period, and on a busy canal this will be quite usual, will weaken the timber above the pivot point of the gate. Hence the beam will fracture at this point or will part company from the gate whichever is the easier.

That’s why in the past gates were always well balanced or if not the situation was quickly corrected. Leaving a gate out of balance for long will cost considerably more than correcting the balance.

Perhaps CaRT should concentrate on the simple but skilled tasks of correction and avoid future expensive repairs or replacements.