Tech that Tuesday–VL3000 Belt Tension

Tech that Tuesday

One of the most common maintenance tasks on most moving lights is checking and correcting belt tension. As a fixture is used and travels in trucks, it’s not uncommon for belts to become too loose or too tight (the former because of truck rides and normal use loosening fasteners, the latter because some belts tend to shrink a little with age and heat exposure). The outcome is similar in both cases – features of the light don’t move right. They might skip steps or simply refuse to move at all. Fortunately, most belt tension problems are easily corrected by the end user.

We’ll be looking at a Vari-Lite VL3000 series fixture today, but many of these principles apply to other moving lights. The VL3000 uses belts to drive pan and tilt, as well as most of the visual effects (color wheels, gobos, the zoom mechanism). It’s a good idea to check on these tensions when performing regular cleanings. You’ll want the following tools:

  • #2 Phillips Screwdriver
  • 5/32” Hex wrench
  • 3/16” Hex Wrench
  • 5/16” Nut driver or open-ended wrench

First let’s look at pan and tilt, as these are pretty easy. We’re concerned with the four screws circled in red. Also note the two springs with red arrows – these will actually set the belt tension for us. The procedure is simply to loosen each screw about half a turn, and then move the head from one endstop to the other a few times to let the springs push the motor to the right position. Then tighten the four screws in order – 1, 2, 3, 4 (This pattern is recommended by Vari-Lite for all spring-tensioned motor mounts, the first screw is always on the opposite side of the springs).

Next up is pan. We’ll need to set the fixture on its side for this. I like to lay it down so that either the back or the front of the head is resting flat on the table. Be careful of the power cable, if you’re working on a fixture without powercon.

Once you’re looking at the bottom, remove the four socket cap screws holding the bottom plate on.

I’ve marked this photo the same way, and the procedure is much the same. You’ll loosen the four socket-cap screws holding the motor mount in place about half a turn, then rotate the pan back and forth a few times (you’ll need to carefully set the fixture back up, it’s fine to do this without putting the bottom cover on, but be careful, the studs for the screws can scratch up a work surface easily).

With that done, your belt tensions are correct and pan and tilt should be working normally.

Other belts in the VL3000 series are not automatically tensioned, so there’s a little bit of a ‘feel’ required. We’ll look at the belts in the color module, as I frequently find belts that are much too tight, causing the wheels problems when moving fast.

The color motors are mounted with two screws each – one screw inserts directly into a threaded portion of that black plate, the other screw goes through a slot in the plate into a Keps nut. This slot allows the motor to rotate to adjust the belt tension.

The belts should not feel completely taut – there should be a little bit of give, but not so much that the belt is able to move around the pulleys without engaging the teeth. If you don’t feel any give when you press on the belt, it’s too tight and may cause problems down the line. Simply loosen the screw going into the Keps nut (a right-angle screwdriver and combination wrench is useful here) and allow the belt to slacken. Very gently pull the motor back against the belt (use one finger and just pull hard enough to take all the slack out of the belt) and tighten the screw. The wheel should spin freely without too much resistance, but shouldn’t wobble back and forth independent of the pully.

Remember that these ideas are pretty universal – if you have a color or gobo wheel in a fixture that’s not moving quite right, belt tension could be to blame (some fixtures use a gear-driven arrangement and in most cases those are not very adjustable, but belts are very common). Sometimes problems can be cleared up with a very minor adjustment in tension, so don’t be afraid to experiment a little while troubleshooting – there are not very many instances where a belt that’s a little bit too loose or too tight will cause damage to a moving light.