What do outriggers on scissor lifts do?
Outriggers are short metal rods that are mounted on the corners of a scissor lift, also on cherry pickers and cranes, which help the lift stay stable.
If the platform on a scissor lift is low down, and the machine is on solid ground, then you usually won't need the outriggers. However, if it's at full extension and goes very high, or if it's on soft ground or you're exposed to high wind, then you can use outriggers to keep the scissor lift stable. When you're 45 feet up in the air, you'll appreciate it if the platform you're on doesn't swing around in the breeze or tilt when you move around on it.
With the outriggers down, they help spread the contact footprint of the scissor lift, as it is now standing on both it's wheels and the feet of the outriggers. This gives more stability and reduces the chance of the lift tilting if anything should go wrong.
Sometimes you need to use outrigger boards under the outriggers. These spread the load further, and are useful where the outriggers may mark the floor, e.g. if you're working inside, or on laid stone flooring outside, or if you're on soft ground and want to make sure the foot of the outrigger doesn't sink in to the ground, which will give you no benefit as it won't help stabilise the scissor lift.
Outriggers come in different designs. Some fold out from the side of the machine, others are mounted direct to the chassis. The older or cheaper machines may need you to wind down each outrigger until it's pressing firmly against the ground. The better machines have hydraulics or motors to push the outriggers down for you, and can sense when they're at the right length so you don't have to adjust them by hand. The best machines have independent, self levelling outriggers, which means electronics on the scissor lift automatically set each outrigger to the correct level for maximum support for where you are trying to work.
The best manufacturers of scissor lifts are Haulotte and Skyjack. If you have a short term job and want to hire a scissor lift, I recommend checking out these guys, they've always been very good for me.
Malcolm Brown has worked fixing a variety of machines, including forklifts, racing karts, cherry pickers and scissor lifts. He has a blog coming out soon.