A door hit by a vehicle is a common cause of disruption in industry. Which is why preventing impact damage is worth the effort.
At Vänerhamn in Karlstad, a forklift or a lorry hit one of the three vertical fabric doors every year before it had finished opening. Replacing the bottom beam cost real money. On top of that, the forklifts had to detour through other doors until the damaged one was repaired. Since switching to side-opening bi-folding doors in 2022, Vänerhamn has not had a single impact.
It sounds odd, but It's true.
"Sectional overhead doors get broken more often than doors that open sideways," says Lilie Jansson, formerly a salesperson at PBAB Svets och Portservice in Karlstad, a company that services and repairs hundreds of doors of every kind each year.
Part of the explanation lies in how we judge distance.
Vertical distances are harder to judge than horizontal ones
In the middle of the 1800s the German mathematician Johann Joseph Oppel set out to find whether we really see the world as it is.
Back then most people believed the eye worked like an exact camera. Oppel suspected our vision was far from perfect.
His experiments showed that our ability to estimate the length of a line depends on how that line is presented to us.
Later research illustrated the point with what became known as the vertical-horizontal illusion.
Most people looking at this illustration see the vertical line as 10 to 20% longer than the horizontal one. Our ability to judge vertical distance is limited.
Our eyes sit side by side, and our field of vision is considerably wider than it is tall. Roughly 200 degrees across and 130 degrees up and down. In practice, our visual system is optimised for horizontal orientation.
We are simply better equipped to judge how wide an opening is than how tall it is.
What happens on the shop floor
A forklift driver approaches a sectional overhead door. The vehicle breaks a photocell and the door starts to lift. The driver slows down or stops, and tries to judge when the opening is big enough to pass through.
Most of the time that judgement is right. Under time pressure the margins shrink. The vertical opening looks big enough when it is not. That is when the risk of hitting the door goes up.
Swap the sectional door for a bi-folding door that opens sideways, and the driver gets to rely on the sense he is better at, judging distance across.
That is an important part of why bi-folding doors do not get hit as often as sectional overhead doors.
The driver's view upwards is often blocked
The other explanation is simpler. A forklift driver's view upwards is often blocked by the mast. Klas Garp, operations manager at Vänerhamn, describes exactly that in this case study.
And fewer impacts mean fewer repairs, better reliability and lower service costs over time.