Update 4

• Escalator Brushes
• Dwell Time & Headway Time
• Hydraulic Valves

Among the after-market products that offered for escalators are brushes installed at the skirt of the escalator. These brushes are intended to discourage people from standing too close to the skirt, thus reducing the risk of side-of-step entrapment.

This product has been on the market for several years. Prices in the range of $5000 and up are often quoted.

The brushes are offered as a “safety enhancing device”. This puts the owner in a difficult position: if he does not install them and there is an accident then questions are asked.

This is a common sales ploy in all industries. A company writes a letter saying "buy this and your unit will be safer". If the customer buys it, the company makes money. If the customer does not buy it, the next time there is an accident, the company throws its hands up and says "I told you so". Under these conditions it is often difficult to separate a valid safety upgrade from those of doubtful value.

It is questionable how effective these brushes are. One accident that comes to mind is that of a little girl who dropped something on the escalator and reached down to pick it up. Her fingers got caught in the step-skirt gap - which was reported to be about half an inch. Difficult to see how the brushes would have helped. There have been accidents involving soft shoes (tennis shoes, sandals and such) getting caught between the skirt and the step. Possibly the brush might have kept the shoes away from the gap. It is doubtful that the brushes would have any effect on the "self-induced" accident resulting from playing on the escalator.

There is no good data to show brushes do anything to reduce accidents. All of the information we have received over the years has been anecdotal. There are remarks such as "Brush guards, which have been used extensively in Europe, have also been quite successful". No numbers, no sources. Over the past fifteen years we have never seen nor heard any hard data. There is a lot of "circular" quoting. Somebody in New York quotes a report written by somebody in London which quoted a report written by somebody in Washington which quoted a report written by someone in New York. All this leaves the impression that there is a mass of data and opinion out there when in fact there is no hard data.

In all of this discussion it is important to remember that the data has a low level of reliability. The 1994 Consumer Protection data was "estimated". There was no information as to how it was collected. Did someone call the local hospital and ask the emergency personnel how many people were treated for escalator accidents? And then multiply this in some fashion for all the hospitals in the country? How would one get this type of information? How would you go about it? But this report has been quoted so often that by now it has the status of absolute fact.

Although the available statistics are highly suspect, it appears that escalator step entrapments represent about 20% of the escalator accidents. Rounded up, the step-skirt entrapment would happen on an escalator once every 100 years. What percentage of the step-skirt accidents would be eliminated by using brushes? No one seems to have any idea.

The London tube installed these brushes some years ago and the person in charge of the brush installation project at the London transit authority wrote an article in which he said that accidents had been reduced significantly. Since he was the person who decided to install the brushes it is just possible that he might not have been eager to tell the world that they were useless (after the expenditure of much money). He gave no numbers.

The side-of-step entrapments proceed, in all of the instances that we are aware of, from excessive gaps between the step and the skirt. It would be more effective and less expensive to pay more attention to maintaining this gap to a minimum rather than add brushes to try to discourage people from having their feet close to the step edge.

It is always desirable to do whatever is possible to improve safety. However, there is a difficulty with the escalator brushes because: (1) there is no good evidence indicating to what extent, if any, they reduce accidents; (2) they are expensive.

Dwell time refers to the time that the doors stay open in response to a hall or car call. This is the same for all floors.

At the main floor there is an additional time inserted by a separate circuit or by other software that is designed to keep the doors open to allow for loading or to provide separation in time from the departure of a previous elevator. This is referred to as "loading time" or "headway time" or "dispatch time".

The headway time, in most systems, varies depending on the traffic pattern. On down peak, for example, there is no good reason to delay the cars at the main floor so the headway time is set to zero. On up peak, the time may be in the order of 20 to 30 seconds subject to the dispatching algorithm. Some systems attempt to regulate the spacing of the elevators by calculating the average interval between cars arriving at the main floor and then trying to maintain that interval using headway timing. The idea is to provide a more or less even level of service and to prevent "bunching" of cars.

In more recent systems the principal idea seems to be to place cars in the hoistway subject to the expected hall calls. The cars are "zoned". The first available car - meaning a car without car calls or other prior assigned task - will go to the first zone, which is almost invariably set as the main floor. The other zones will be assigned more or less in proportion to number of cars and number of floors so as to get have a car handy to any hall call. This zoning can be dynamic or fixed. Typically, it is fixed since the nuances of dispatching system are generally perceived in practice to be of only marginal benefit. The zoning is, of course, adjustable.

There are innumerable other permutations on this theme including number of cars at main for up peak, heavier up or balanced and zoning predicated upon a high profile tenant.

As well as the hydraulic machine internal speed control valves, hydraulic elevators have valves in the pit and at the hydraulic machine.

The check valve is a one way valve, essentially a swing door which will open when pushed in one direction and will close more firmly when pushed in the other direction. The check valve is immediately ahead of the hydraulic pump output. It prevents oil flowing in reverse through the pump but allows it to flow in the proper direction when the pump is activated. To start the elevator moving in the up direction, the pump starts turning and when the pressure developed by the pump exceeds the static pressure developed by the weight of the cab on the piston (this pressure being transmitted through the cylinder and piping back to the hydraulic machine) the check valve is forced open, the oil flows through and pushes the piston and elevator up.

It is desirable to have an overspeed valve and a shut-off valve in the pit.

The shut-off valve in the pit is in addition to the one at the hydraulic machine. The pit shut-off valve is a convenient maintenance tool in that it cuts off oil flow close to the cylinder which makes some maintenance operations easier. These shut-off valves typically are gate valves although ball valves are sometimes used.

The gate valve is the one we are all familiar with as used on garden hose outlets.

The ball valve consists of a ball with a hole drilled through its centre. When the ball is turned so that the hole lines up with the pipe, oil flows; when the ball is turned 90 degrees, the oil flow is cut off.

The overspeed valve is designed to close the line when the oil pressure drops abruptly. This valve was introduced primarily to deal with the perceived danger of flexible oil lines breaking as opposed to rigid pipes which are thought to be unlikely to rupture. In fact, it is a useful additional safety device.