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.   hunkin






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 it. 

 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 keepers. 

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 steel nearby.   

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 and working.

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 finished.

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.





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