Seasonal Elevator Rope Contraction and Expansion
By Rob Isabelle
Recently, a client approached us regarding the effect humidity has on elevator hoist ropes. We investigated and discovered that while humidity does effect the rope lengths, these variations are minimal and do not impact the operation of the equipment provided rope maintenance is being performed.
Our report has further details regarding this incident.
As requested, we have reviewed the elevator issues at the client site. More specifically, we understand that the elevator service provider advised in July 2020 that a high humidity level in the elevator hoistways was resulting in abnormal hoist rope length contractions thus recommending the installation of dehumidifying equipment.
While we agree that hoist rope lengths will vary depending on humidity levels inside the elevator hoistways, these variations are normally minimal and do not impact the operation of the elevator equipment as long as rope maintenance is performed.
Our approach was as follows:
We supplied and installed hygrometers inside the pit of each of the three main elevator banks.
Over a six-week period, we visited the site and measured the following:
relative humidity
temperature
rope length variation
A refence point on the elevator pit equipment (bottom section of the compensating sheave[1]) was identified during our first survey and variations from this point was measured on subsequent inspections. The variations were measured with the elevator located at its next lowest landing thus at the point nearing maximum hoist rope length.
Results have been summarized as follows:
Based on the above, we can conclude as follows:
An increase or decrease of 30% in hoistway relative humidity (which normally occurs depending on the season) can impact the hoist ropes by as much as one inch for every ten floors of building. This is evidenced by a 2.9” increase of traction rope length over 300 feet on car 6 and 3.5” over 360 feet on car 9 (based on a close to thirty percent humidity swing).
Different rope specifications can behave differently. As observed on car 1, 500 feet of rope length only expanded by 0.5” over a 30 percent humidity drop. Car 1 is a higher speed elevator when compared to cars 6 and 9 thus possible that different rope specifications were used. Within these rope specifications, the type of core used (fiber core versus wire rope core) will have an impact. We suspect that car 1 is equipped with a wire rope core which will not be subject to much fluctuations when compared to a fiber core rope.
Hoistway dehumidifiers are not necessary.
Recommendation
The client had indicated that a number of June and July 2020 call backs were the result of the above variations. In these cases, the compensation sheave limit switch would trip due to excessive contraction of the hoist ropes. These call backs could have been eliminated through a rope length adjustment to ensure that the limit switch would reside closer to the upper limit cam during the winter months (when humidity is less) thus allowing variations as humidity increases in the summer months. The distance between the upper and lower switch activation cams is 17 inches which can allow for considerable variation. Please see picture below:
