SLHS 2. Lock Gate Construction

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1. Picture Vault:  Stratford Canal

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Modern Gate Manufacture

Rather than write about it here’s two videos showing lock gate manufacture:

YT: CruisingTheCut: Bradley Gate Factory (10m08)

YT: FoxesAfloat: Stanley Ferry gate Factory (12m53)

Using The Refurbishment Of Lock 53 On The Stratford Canal As An Example

Manufacture Of Lock Gates

Custom Replacement Gate Sizes

It would be utopia if all gates were identical. Understandably, when the canal was built, many, but not all, would have been made to the same pattern. However, as the centuries pass earth heaves and slips, canal layout changes and/or lock sizes are modified so each new lock gate has to exactly match the one that it is replacing -just like your shiny new double glazed windows that replace your old wooden framed units - none is identical.

Each gate starts as a bundle of oak trees and a lorry load of iron ore. The former are cut and rough machined to dimensions larger than required and seasoned over several years to allow for shrinkage or warp of the original green wood. The latter is smelted, cast and machined into shoulders, flanges, paddles and strengthening plates. At the time of writing (2024) all components are delivered to one of two gate making factories owned by the British Canal & Rivers Trust in Stanley Ferry near Wakefield (large gates) or Bradley in Birmingham (up to 3 tons).

At the factory, teams of staff perform carpentry on a massive scale. There is nothing lightweight about anything so cranes are used in every process.

Static Forces On The Gates

A simple door in a house needs to be fairly strong yet is actually quite heavy. However, a lock gate has to be able to withstand significant forces. But exactly how big are these ?

1. Width between Coins =
   

2. Height of water upstream of upper gate =
   

3. Height of water upstream of lower gate =

1. 7ft = 2.13m
   

2. 5ft = 1.52m
   

3. 12ft = 3.66m

Dynamic Forces On The Gates

These are remarkable forces. However, they are not the final story. Again, those darn boaters are a further problem. It would be all very well if everyone politely and gently opened and closed each gate as they went about their business. When time is money unfortunately, not everyone cares for the furniture as they should. Those men in a rush to get paid (and sadly the skippers of amateur leisure craft these days) did not always wait for a gate to be fully shut before operating the paddles. They would let the rushing water to do that final bit of work for them and it would take a moment for the massive gate to accelerate as it started to turn -more water would flood in. Then smash ! It would slam into the coins. That bang would be because of the rapid deceleration of the mass of the gate which will damage both the gate and the lock. No wonder they need to be extremely strong.

The heyday of canal construction occurred 50 years before the first experiments in photography by William Fox-Talbot and so to see how gates are made we will look at the methods used by the British Canal & River Trust nowadays..

Design Of The Gates

This is a continuation of the previous page on Lock Construction

Top Gates

In order to raise or lower a boat in the lock water must enter and leave it in the correct order. A top gate will never have an inlet in it otherwise the water would cascade over the boat and sink it. Instead, it will flow through a culvert in the brickwork around it.

Bottom Gate

The bottom gate may operate in the same way but it is cheaper to merely have an opening in it below water level.

Paddles

By whichever route, the flow of water is limited by a large sliding flap called a paddle, that runs in a groove. The paddle is pushed by the water against smooth sliding surfaces and is raised or lowered by a rack & pinion via a vertical shaft. The end of the axle that the pinion turns on is square so that a handle, called a Windlass (see far right), can be attached to turn it. Considerable force may be required to do this. A simple rachet prevents the mechanism from falling under its own weight until required to do so (keep fingers out !)

Calculation Of The Force On The Gates

I was once asked why the handles are detachable. Firstly a spinning handle is a safety hazard and so only people who know what they are doing should be operating the system. Of course, the second reason is to stop children from draining the whole canal as a prank.

Crossing Point

If the gate is to be used for traversing the lock then walking boards and a handrail will also be added. Let’s now look at the forces that these gates must bear..

Practical Example

Fortunately at time of writing Lock 53 in Stratford town centre is being completely refurbished. See next page in our practical mini-series:    3. Refurbishment Of A Stratford Lock

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