Limit Switches - DDM Garage Doors Blog

Posted August 28th, 2020 at 5:02 pm by Dan Musick

Garage door opener limit switches electrically limit the door travel as it opens and closes. These switches “tell” the opener or operator when to stop.

The most common limit switch comes in most of the Sears, Chamberlain and Liftmaster openers made since the early nineties.

An interior view of a limit switch.
As the center shaft rotates, the metal contact slides between the open and close contacts.

Another common type of limit switch found on commercial and industrial operators use a limit shaft with two plastic nuts, as shown on this LiftMaster operator.

An image of the inside of a LiftMaster operator showing the limit nuts and switches.
The limit nuts, marked in yellow, slide to the left and right to activate the open and closed switches.

PowerMaster operators use plastic nuts with a design similar to the LiftMaster limit assemblies.

A closer look at the PowerMaster operator's limit switch.
The plastic nuts on the far right slide between the limit switches to stop the door in the open or closed position.

The older Edko operators used wooden nuts that slid between the open and closed switches.

A view of an older Edko operator with a red arrow pointing to the wooden nuts.
Notice the shaft on the left with the two hex nuts. As the operator runs, the nuts slide to the left or right to stop the travel of the door in the open or closed position.

Link operators used plastic nuts sliding along the limit shaft.

The view of a limit shaft with plastic nuts sliding on it for a Link operator.
On Link operators, the sliding nuts on top activate the limit to stop the door travel.

McKeon operators use a similar system with round plastic nuts. A steel plate prevents the nuts from turning on the shaft as the shaft turns.

An image showing the limit switches on a McKeon operator.
Here the limit switches are on the bottom. A limit chain and sprocket turn the limit shaft to move the limit nuts.

The older Crane operators used limit assemblies with two nuts.

A view of the plastic hex nuts traveling between the limit switches.
Crane manufactured limit assemblies with plastic hex nuts that travel between the limit switches. Below you can see the sprocket and chain that turn the limit shaft.

Allister, Allstar, and MVP openers normally used chain dogs to stop the door as it opens and closes.

An image of an opener that using dogs to trigger the limit switch.
As the door operates, the chain moves in either direction to open or close the door. The dogs trigger the limit switches.

Below you can see where the limit dog has moved an arm. The arm then activated the switch to stop the door travel. The limit dog can be moved along the chain by loosening the screw that secures the dog to the chain.

A view of a MVP opener using chain dogs to activate a stop switch.
MVP openers have chain dogs that activate a stop switch.

Over the decades, Genie has used switches mounted to the track.

A view of the Genie screw drive system using a trolley with mounted switches.
The Genie screw drive system moves a trolley to open and close the door. Installers mount the switches, so the trolley throws the switch at the open or closed position.

The older Scientific operators also used track-mounted switches to stop the door when it opens and closes.

An older Scientific operator using track-mounted switches to stop the door.
As the shaft turns, the trolley slides along the center tube, and it activates the track switches when the door is fully open or closed.

The older Vemco operators used a small tube that ran the full length of the operator rail.

An older Vemco operator using a trolley with arms at each end of a tube located in the operator's rail.
When the trolley reached the open or closed position, the trolley would raise an arm that was attached to the tube. When this end turned, an arm at the other end of the tube would turn and trigger the open and closed limit switches.

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