Elevator Overspeed Safeties Device Types

In our last blog entry, we noted the application of mechanical safeties devices used to halt an elevator's downward motion should that speed exceed a predetermined value. That value is determined by desired (known as contract speed), weight and distance. Today, there are generally three classifications used for these devices: A, B, C. There are also some variations of other types. If you'd like more information on other types and applications, just drop me note or respond to this blog entry on CR4 and I'll be happy to help.

This information is drawn from USA code standards and may not apply elsewhere. Elevator safeties are basically simple and haven't really changed much in the last 100 years. Elevator guide rails have evolved from hardwoods to steel and the safeties jaws have changed to reflect that. Guide rails are vertically stacked and fastened to the shaft walls (hoist-ways) via brackets anchored in the wall. Think of train tracks vertical with flat blades instead of the slightly rounded type used with trains. Then elevator guides wrap around the rails and "guide" the elevator as it travels up and down.

Guides are either slide-type or rollers. The safeties are usually mounted below the guides and also wrap around but do not touch the guides unless activated. The guides and safety assemblies mount to a beam structure referred to as a "Safety-Plank" on traction elevators. This safety-plank spans the width of the elevator cab assembly and supports all the good stuff: platform, guides, and mounting point for related equipment.

The most common safeties we are discussing here are located under the elevator and clear the guide rails by fractions of an inch. How much clearance depends on the manufacturer's design. Common is 1/8" or more on each side of the rail. There are variations to this as well, with some safeties mounted on top of the elevator and others (as in The Empire State Building) with safeties on the top and bottom of the elevator for more gripping force.

We will look at Type A "Instantaneous" Safeties in this article and progress through the other type in future blogs. Type A safeties are allowed for use on elevators not exceeding 150 feet per minute (fpm), 0.75 m/s. "Instantaneous" means that they are applied without appreciable delay when triggered, either by an over-speed governor or free-fall condition as would result from the suspension means failure. The type A I am most familiar with have knurled steel rollers contained in a housing on each side of the guide rails that allows upward movement of the rollers when activated by pulling up and toward the guide rail via an attached lever to a rope.

Pull the rope upward. Lever mounted, the safety-plank moves the rollers up into the guide rail on each side, resulting in clamping the safeties mounted to the safety-plank to the vertical rail. I tried to attach a drawing I made of these safeties, but no-go on that; I can e-mail it to anyone who's interested. (Meanwhile, I'll contact blog administration and see what we have to do get this drawing posted.) I can testify that these safeties when maintained properly stop the elevator "right now!" Even at the slow speed of 150 fpm, it is a jolt.

I've used these same safeties while working on high-rise traction elevator installations. We use a portable working platform suspended by 3/8-inch cable - the same as commonly used by window washers - but we use them inside a hoistway. The platform has Type A safeties that are controlled by a spring-loaded lever on the platform. This lever keeps constant pressure upward on the knurled rollers so they contact the guide rails. The lever has to be manually forced down, which moves the rollers down and away from the guide rails when moving the platform, usually by foot pressure. If an over-speed or uncontrolled descent happens, the foot is lifted from the lever and the safeties engage.

Every morning, after any break or lunch time during the day, the safeties are inspected and tested. I know of a few incidents, one fatal, where the safeties actuation lever was tied down because the operator apparently didn't want to hold pressure constantly while operating the movement controls on the motor-rig. The suspension failed and he rode the platform down while frantically trying to untie the lever. Too little, too late.

Lifelines are required, one per person on the platform. These are often not attached to the fall protection harnesses, as was the case here. Smart and safe elevator people use two or more redundant safety devices to protect themselves. But we only have one brain. Next we'll look at Type "B" safeties. Contact me or respond to this blog entry with questions or ask for more information. Am I technical enough, or am I lacking? Are my words conveying what I see? I need the feedback - please.

Joe