Canal Lock Gates

[MtB]

With a single gate, the all the force created by water pressure is in line with the lock side, while with a mitre gate the force is divided, partly in line with the lock and partly at right angles and thus pushing into the lock sides.

 

I disagree. If you draw the vectors I think there will be the same force bearing along the length of the lock walls whatever format gate, but an additional outwards force on the hinges of the mitre pair, as a result of the mitre.

 

MtB

[Scholar Gypsy]

 

I disagree. If you draw the vectors I think there will be the same force bearing along the length of the lock walls whatever format gate, but an additional outwards force on the hinges of the mitre pair, as a result of the mitre.

 

MtB

 

Yes, that must be right. A simplified example is worked out in this thread from 2013 ...

 

Is a guillotine a gate? Just being picky but to me a gate has to swing.

 

Well the instructions on the locks certainly call them gates. And a gate valve goes up and down, rather than swinging.

 

On the other hand, the things at the other end are, of course, called V-Doors not gates ....

[FadeToScarlet]

Is a guillotine a gate? Just being picky but to me a gate has to swing.

Good point; most call them gates; I just call them a bleeding nuisance!

Yes, that must be right. A simplified example is worked out in this thread from 2013 ...

 

Well the instructions on the locks certainly call them gates. And a gate valve goes up and down, rather than swinging.

 

On the other hand, the things at the other end are, of course, called V-Doors not gates ....

Or pointing doors, the other phrase I've seen on the Middle Level and Nene.

[Pluto]

 

I disagree. If you draw the vectors I think there will be the same force bearing along the length of the lock walls whatever format gate, but an additional outwards force on the hinges of the mitre pair, as a result of the mitre.

 

MtB

Peter Barlow, in his paper on the strain in lock gates published by the ICE in 1836, talking about whether to have straight or curved mitre gates for docks, states:

 

A common straight gate is exposed to two strains; one a transverse strain, produced by the weight of water at right angles to its surface, which is equal to half the weight applied in the middle; the other a strain in the direction of its length, produced by the pressure of the opposite gate upon its extremity. This latter strain, if the salient angle was of 45 degrees, or the gates stood at right angles to each other, would of course amount to half the weight on the opposite gate, so that at this angle a lock gate has, in addition to the transverse strain, and equal strain in the direction of its length.

 

So as the angle of mitred increases, so does the strain tending to push the gates into the wall. You haven't changed the overall force, which equates to the depth of water in the lock. In an infinitely long mitre gate, all the thrust would be into the lock wall.

[Tacet]

How many locks have a pair of top gates and single gate at the tail end?

 

 

 

If you mean canal locks, 1, Hall Green, but I like Scholar Gypsy's answer because I hadn't thought of it!

 

The guillotine gate answer occurred to me as well as Hall Green - but is Hall Green the only two-top and one-tail all-casement gated lock? I had a feeling there was a second example - but I can't think where.

 

Do you know why Hall Green is so arranged, Patrick? It has a modest rise - but that doesn't seem to be an answer in itself.

[magpie patrick]

 

 

 

The guillotine gate answer occurred to me as well as Hall Green - but is Hall Green the only two-top and one-tail all-casement gated lock? I had a feeling there was a second example - but I can't think where.

 

Do you know why Hall Green is so arranged, Patrick? It has a modest rise - but that doesn't seem to be an answer in itself.

 

I don't know why, and I doubt it's a matter of record. The shallow rise of stop locks tended to mean they had single gates at both ends (there were once rather more than there are now).

 

I've pondered this a lot, and have reached a rough guess although it is just that. There is a second chamber below the one you now use, which was a lock that could face the other way. Given the one foot fall of Hall Green the idea of ever having to lock down from the T and M seems an unlikely one, but I have heard from a BW engineer that the T&M summit was lowered by several inches at some point, perhaps in the 60's or 70's to increase headroom for Harecastle Tunnel (Wikipedia says the same thing, although that doesn't make it true). Bear in mind the same subsidence which lowered the roof would also have lowered the bed.

 

Returning to facts: there is another chamber facing the other way, and the other 12 locks on the Macclesfield have double top and bottom gates, so it's the single bottom gate which is "non-standard" not the double top ones. Is the lower gate single because of the second chamber, and the long vanished gate facing the other way just downstream? I haven't heard a better theory although 'm not totally convinced by mine

[Keeping Up]

I'm pretty sure the both ways theory is correct for Hall Green. I vaguely remember hearing at the time about the level of the T&M being lowered because of the subsidence in Harecastle tunnel, so I imagine it was very likely that originally there was the potential for sometimes one canal to be higher and at other times lower. Whether or not it was actually reversed is irrelevant, the possibility of it happening would be sufficient to justify the construction of a both ways stop lock, although I would have thought that other methods, such as at Dukes Cut or at King's Norton, would have been cheaper.

[luctor et emergo]

On a different tack, I stopped on my way to Swansea at 14 locks. Intriguing setup of sets of staircase locks, with an very interesting way to fill the side ponds. And a fascinating feature in one of the locks, which has shelfs on both sides, which look like they may have been used for 'dry docking'. I'll post the pics when I have a better connection.

[magpie patrick]

On a different tack, I stopped on my way to Swansea at 14 locks. Intriguing setup of sets of staircase locks, with an very interesting way to fill the side ponds. And a fascinating feature in one of the locks, which has shelfs on both sides, which look like they may have been used for 'dry docking'. I'll post the pics when I have a better connection.

 

Please post! More probably a passing place as the bays are in the middle of a "three rise" in a flight made up mainly of "two rises". (they aren't true staircases). Another debate to get into

[Pluto]

Vertical lifting gates are common in mining areas where it can be very difficult to get mitre or other gates to seal properly. When Pagefield Lock was built in the early 1900s in Wigan. it was designed to have a lifting gate for this reason. You can find a number of such locks in the mining area around the Ruhr in Germany. This is one on the Wesel-Datteln Canal, with a friend's boat, the Amoebe, entering the lock after loading coal.

[luctor et emergo]

 

Please post! More probably a passing place as the bays are in the middle of a "three rise" in a flight made up mainly of "two rises". (they aren't true staircases). Another debate to get into

Patrick, ive uploaded some on my facebook, have a look.

[IanD]

Peter Barlow, in his paper on the strain in lock gates published by the ICE in 1836, talking about whether to have straight or curved mitre gates for docks, states:

 

A common straight gate is exposed to two strains; one a transverse strain, produced by the weight of water at right angles to its surface, which is equal to half the weight applied in the middle; the other a strain in the direction of its length, produced by the pressure of the opposite gate upon its extremity. This latter strain, if the salient angle was of 45 degrees, or the gates stood at right angles to each other, would of course amount to half the weight on the opposite gate, so that at this angle a lock gate has, in addition to the transverse strain, and equal strain in the direction of its length.

 

So as the angle of mitred increases, so does the strain tending to push the gates into the wall. You haven't changed the overall force, which equates to the depth of water in the lock. In an infinitely long mitre gate, all the thrust would be into the lock wall.

Unfortunately the force pushing the gates into the lock wall would then be infinite -- which not even good old English oak can withstand...;-)

 

-- actually if the gate is infinitely long the force at right angles to the gate would be infinite, so the force into the lock wall would be infinite squared...

Edited March 17, 2015 by IanD

[luctor et emergo]

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[Machpoint005]

Peter Barlow, in his paper on the strain in lock gates published by the ICE in 1836, talking about whether to have straight or curved mitre gates for docks, states:

 

A common straight gate is exposed to two strains; one a transverse strain, produced by the weight of water at right angles to its surface, which is equal to half the weight applied in the middle; the other a strain in the direction of its length, produced by the pressure of the opposite gate upon its extremity. This latter strain, if the salient angle was of 45 degrees, or the gates stood at right angles to each other, would of course amount to half the weight on the opposite gate, so that at this angle a lock gate has, in addition to the transverse strain, and equal strain in the direction of its length.

 

So as the angle of mitred increases, so does the strain tending to push the gates into the wall. You haven't changed the overall force, which equates to the depth of water in the lock. In an infinitely long mitre gate, all the thrust would be into the lock wall.

 

The strain cannot be equal to 'half the weight' of anything: strain is dimensionless (extension per unit length) whereas stress is force per unit area. Apart from that, Mr Barlow was right.

I think he means that the weight of water is equal to half the weight applied in the middle, though.

Yes, I know, but it annoys me when 'strain' is used as a definition of 'stress' in a crossword clue, too.

[Pluto]

 

The strain cannot be equal to 'half the weight' of anything: strain is dimensionless (extension per unit length) whereas stress is force per unit area. Apart from that, Mr Barlow was right.

I think he means that the weight of water is equal to half the weight applied in the middle, though.

Yes, I know, but it annoys me when 'strain' is used as a definition of 'stress' in a crossword clue, too.

I just used the original wording, and did wonder if the technical standardisation of the word had been thought about at the time. Old documents such as that do need a little 'interpretation' given modern usage. Despite having been involved with technical history for so long, I am still unsure about where to use original descriptive words, or modern ones, and using both can sometimes be just as misleading.

[FadeToScarlet]

-- actually if the gate is infinitely long the force at right angles to the gate would be infinite, so the force into the lock wall would be infinite squared...

I believe the force would still be infinity, as infinity times infinity is still infinity.

[David Mack]

Whereas infinity divided by infinity can be more or less anything you like!

[Scholar Gypsy]

Please see this stylised model of a mitred lock gate.

 

The only assumption made is that the bottom cill does not exert much force on the lock gate - that would be easy to achieve if one used rubber seals to make it watertight, not the pressure of the gate on the cill.

 

Interestingly,

 

• the force at the hinge along the lock's length is a constant, not affected by the angle of the gates;
  
• if the angle of the mitre is 45 degrees (so the gates meet at a right angle) then there is no transverse force on the lock walls at all. I wonder if what the paper about strains etc is trying to say?

Edited March 17, 2015 by Scholar Gypsy

[MtB]

Yes, I know, but it annoys me when 'strain' is used as a definition of 'stress' in a crossword clue, too.

 

Yes thanks for clarifying that. I meant to earlier but forgot. Stress is the force, strain is the change in size resulting from the stress.

[Pluto]

This is taken from Waterbouwkunde, vol 3, p82, published in 1968, and shows the forces acting on a mitre gate. The size of the force acting into the quoin alters with an increase in mitre angle, while its direction alters at the same time.

[Scholar Gypsy]

Thanks, my Dutch is not that good, but the forces diagram is the same as in my posting above, I think.

 

He has resolved the quoin force along and perpendicular to the gate; I resolved it parallel to and across the lock. The answer should be the same. (2s/d = tan theta etc).

Edited March 18, 2015 by Scholar Gypsy

[Scholar Gypsy]

Interestingly, some more trigonometry shows that angle CBD = angle ABC. This rather surprised me.

 

As a special case, when the mitre angle (ABC) is 45 degrees, then the force R_B is parallel to the lock side (and is equal to the force (well half of it, of course) that would be exerted on a single gate stretching right across the lock.

[Mike Todd]

I just used the original wording, and did wonder if the technical standardisation of the word had been thought about at the time. Old documents such as that do need a little 'interpretation' given modern usage. Despite having been involved with technical history for so long, I am still unsure about where to use original descriptive words, or modern ones, and using both can sometimes be just as misleading.

In structural engineering (and other related mechanical disicples) the difference between stress and strain goes back a very long time. I sometimes fid it useful when people refer to them in common parlance to expand by saying that stress is the internal response to external forces.

[IanD]

I believe the force would still be infinity, as infinity times infinity is still infinity.

I was making a mathematical joke...

Whereas infinity divided by infinity can be more or less anything you like!

Nope, it's one :-)

[Pluto]

In structural engineering (and other related mechanical disicples) the difference between stress and strain goes back a very long time. I sometimes fid it useful when people refer to them in common parlance to expand by saying that stress is the internal response to external forces.

Those who built British canals were certainly not thinking in terms of stress and strain, apart from ear-ache from their canal's proprietors, as they were craftsmen without academic education. The perceived correct use of technical words only really came into general usage in Britain with the growth of academic technical education in the mid-nineteenth century, being more widely used from the 1820s to 1830s. The technical design of skew bridges is a good example, with mathematical descriptions of the forces only being developed in the 1820s. There had been earlier examples, but they were built by craftsmen working with inherited practical knowledge, and very basic mathematical skills.