When I got an email asking if I would like to make
some gates for London Zoo’s tigers I couldn’t say no. Is a more
glamorous commission possible?
In the past, the zoo had always used horizontal
sliding gates. But the relentless rise in health and safety standards had
lead to the design of an elaborate
new warren of cages for the tigers. The simple horizontal gates would not
fit in five locations and these gates would have to slide vertically.
Initially I couldn’t understand why this was difficult, I only later
realised the problems involved.
The architect and keepers had visited Vienna
zoo which had sophisticated electronic vertical gates. I suspect London
zoo couldn’t afford them so were looking for a cheaper solution when
they contacted me. My initial instinct was that electronics were a bad
idea anyway, because if one failed the zoo’s maintenance team would have
to call out specialists. If the gates were entirely mechanical the
maintenance team would be more likely to be able to mend it themselves.
This is KISS technology – ‘Keep It Stupidly Simple’
The mechanism is simple, but it’s really important
that it can’t fail. For example, I used plain nylon strips for the
slides rather than rollers which could potentially jam. I used polyester
rope instead of steel because I’ve had trouble with strands breaking.
It’s the same sort of fail-safe approach that is used in nuclear
engineering. I presented my drawing, and was delighted that they accepted
With the design worked out, you might think
everything else would be straightforward. In practice, it was only the
beginning of the journey. The first problem was trying to fabricate the
frame accurately. This was important because the channels had to be
completely parallel for the gates to slide smoothly. Every weld distorts
the frame a bit so it’s a real skill working out where to clamp things
and what order to weld everything. Later on I realised it was better if
the gates were a really loose fit so this fussing was mostly unnecessary.
The gates at Vienna zoo were solid to avoid finger
(or rather claw) traps. With 1.5mm thick steel plates on both sides of my
gate I was shocked to find it weighed over 30 kg. It was certainly too
heavy to lift comfortably by rope. The whole thing looked scarily like a
guillotine and Graham, who was helping me, pointed out that I needed the
locking device to keep the gate open as well as to keep it closed. The
keepers had been nervous about vertical gates not because the tiger might
escape, but the gate might fall on it – and I really didn’t want to be
responsible for crushing a tiger’s tail.
I added counterweights, but then the gate wouldn’t
close when the rope was released. So I added an extra pulley. This doubled
the mechanical advantage. The rope is pulled twice the distance but with
half the effort. On day three, we tested the prototype. It just about
worked but the lock wasn’t easy to use and the gate was still stiff
sliding. After another day it was ready to take to the zoo to show the
At the zoo, a Polish builder helped me assemble it
for the trial. He thought the keepers were crazy. ‘We are doubling the
thickness of walls but what does a tiger weigh – maybe 120kg, that’s
much less than a car, its nothing.’
Eventually lots of keepers and officials arrived.
After a short demonstration they tried the gate themselves. A fierce
Scottish keeper kicked it as hard as he could – not leaving even the
slightest mark – very satisfying. Without unlocking the gate, he then
pulled on the lifting rope as hard as he could and managed to move the
locking bars a bit (though not nearly enough to release the gate). His
verdict was that the design was just OK for a big cat, but he wouldn’t
have passed it for a bear. Cats are basically lazy and give up quickly but
bears can persevere for ages.
Despite being passed for strength, the gate failed
its trial by keepers for a completely different reason. I had imagined
tiger keepers would be strong men, but at London zoo the mammal keepers
work as a team and at least two of them are small women. They found my
gate too heavy to lift, even though we had added the extra pulley to make
it easier. (The attitude to lifting things has changed radically in my
lifetime. Coal used to be carried in 100kg sacks and cement came in 50kg
bags. The maximum sack weight was then reduced to 25kg and now workers can
refuse any load if there is a possibility of back injury.)
We couldn’t add more counterweights or the gate
would not have enough weight to close. The only possibility was to
decrease the friction. I assumed the main source of friction was the
slides, but we quickly found the problem was the pulleys. These were
standard yachting pulleys with plain bearings. Fancy yachting pulleys with
ball bearings are available, but the bearings are open. This is fine at
sea but not suitable for a cage where dirt and straw could get in. So we
made our own pulleys with sealed ball bearings. Satisfyingly, these almost
halved the effort needed to lift the gate from 15kg to 8kg.
Two keepers, including their smallest, had agreed to
come to my workshop for the second trial. She passed it as easy enough to
use, and then suggested several sensible modifications, particularly
marking the two positions of the locking rope ‘locked and unlocked’.
My original plan had been only to make the prototype
and then pass the design on to a company to manufacture and install the
final gates. However, as the design had evolved, I had got increasingly
anxious that this could go horribly wrong and decided to install them
myself. To prepare for manufacture, I drew all the parts on CAD, but a
local company called ‘Eastern Hardware’ said they would prefer to have
the prototype in their yard and work from that. This is also how I prefer
to work, but it does require the fabricator being a ‘schemer’ who
prefers problem solving to rigidly following drawings.
I was lucky, I immediately trusted Brian and he did a
brilliant job. We spent two days working together assembling everything
after the parts had been galvanised. Like the Polish builder he also
thought the design was a bit ‘over the top’. He had just watched a TV
program about a New Zealand man who kept tigers as pets. He claimed they
were perfectly docile if never fed raw meat (always cooked meat) and given
a daily bath to stop hormones building up.
The installation was not straightforward. I had to
pack everything I might possibly need because it can take hours going off
site to buy anything. In the past when I often worked away from home my
van was full of useful stuff, but now I’m out of the habit and my van is
empty. Also the tools were not straightforward because, unlike most UK
tools which are 230volt, building sites require 110volt tools and I
don’t have any. I hired some 110volt tools and packed my generator in
case we also needed one of my 230volt tools.
I’m not used to working on building sites. In
advance it was all very formal. The architect’s drawings, the paperwork
for the risk analysis and method statement, the obligatory safety clothing
and the induction lecture. However once finally on the site, everything
was chaos, though fortunately a benign chaos. It was mid winter and after
a wet autumn the site was alternately deep mud or covered in snow and ice.
We had been there less than an hour when we disgraced ourselves.
A tele-handler had driven our gates close to their
location. Here there was a handy pallet truck so we transferred everything
and pulled it the rest of the way. A very bad move. The concrete under our
feet had only been poured the day before so the wheels left an indelible
trail. I had to go and confess. Lots of officials arrived and spent ages
looking at the damage. I was given a strong reprimand so for the rest of
the installation I remained terrified what would happen next.
In fact, the next shock was the cage dimensions.
Although the architect’s drawings looked so precise, in reality the
cages were 50mm lower and a beam the ropes had to pass under was 100mm
lower. The team installing the cages were busy grinding bits off
everything to make them fit, so we had to follow their example. It feels
wrong to drill, cut and weld steel that has been galvanised (dipped in
molten zinc) to protect it against rust. Fortunately the ‘galvanic’
protection works like a battery so oxygen attacks the zinc in preference
to the steel, and this protection extends beyond the zinc to any exposed
It was a slow process though and after three days
(the time I’d estimated for installing the gates) we still weren’t
nearly finished. We returned the following week, having welded up some
extra parts to make things fit. After another two days on site, struggling
through intermittent snow and rain, the gates were finally all installed
The keepers then had to inspect and pass the work.
Even though they’d seen the prototype, they expressed surprise that the
gates were solid. Mesh would have been better so they could see the tiger
on the other side. If we had used mesh instead of solid plate, the gates
would have been so much lighter we would never have had to add pulleys and
reduce friction. How unbelievably irritating!
So despite my initial enthusiasm, the job wasn’t
remotely glamorous. With the winter weather, the installation was quite
miserable at times. And I never even saw a tiger until a month after I’d
Still, I am proud of the gates. Some people do
extreme sports, but my thing is extreme making. I was not the obvious
person for the job, I normally make arty things. In the past it would have
been easy for the zoo to find a company to design and make them. There
used to be thousands of small engineering firms in the UK. But today, the
engineering firms that remain have ‘modernised’ and now work more
formally. Even if willing to take on a small job like this, they would
have charged a lot more money and been less flexible faced with problems
like the wrong size cages. The last generation of traditional engineers
like Brian are retiring, so perhaps the future for jobs like this will lie
with me and other makers. People who do it because they love practical
problem solving and making things.