Home : Technical : Facts & Figures : Haulrope : Negative Cord Angle

What we are looking at is a negative cord angle of the haul rope.

As a lift gets designed, each tower cord angle has a critical relationship with the tower below and above it. For a full load condition on the up-line side of this tower, the haul rope will want to sag away from the tower towards the ground; the tower is lowered so the minimum design sheave weight is maintained to prevent the haul rope from moving out of its groove, (quite common on some lifts). In effect this tower is preloaded so it can produce the proper sheave load in all loaded conditions.

It's possible this design could also be accomplished by putting a compression style assembly at this tower; however this would complicate the tower design as it would have to be built to accept both positive and negative loads.

In each area there are given limits and recommendations for corrections if the limit is exceeded. Haul rope actual safety factor in ski lifts is quite high compared to other wire rope uses. To my knowledge no haul rope has broken while in normal use at a ski area.

This style of tower is an elegant engineering solution - the negative load that the haul rope imparts on the tower is canceled out by the weight of the assemblies and tower components. No support to the ground is needed.

The negative cord angle also preloads the assembly that is in the terminal, this sheave load can become negative when a large load passes the center pivot line of the tower in the image. Again remember each tower has a close relationship with the towers next to it.

Top drive, bottom tension lifts will induce extreme dynamic negative loads on down line terminal assemblies. The drive system will also lift the counterweight at it tries to pull the load uphill.

YAN has many towers that were similar to image but the tower is hinged at the terminal cross arm. The tower just floats on the haul rope! This provides a constant weight on the haul rope regardless of loaded condition.

Definitions:
Cord Angle: the change of angle between the slope of the haul rope and horizontal; measured at the tower cross arm.
Breakover cord angle: difference in cord angles between towers. Also called breakover.
Up-line: the haul rope that goes up the hill.
Down-line: the haul rope that come down the hill. Also written as Dn-line.

Up hill: an object that is up-the-hill from you.
Down hill: an object that is down-the-hill from you.

Dead load: weight from all of the components of the structure and or object. For a lift the haul rope is under tension but no loads are in the carriers. Also written as D.L. or DL
Live load: weight that is added to the structure. Due to the dynamics of a lift it can be negative and positive. Also written as L.L. or LL
Total load: DL+LL

Profile: a side view of lift showing towers, cord angles, loads and ground.

Example of a tower load "call out" from profile of Ski Bach image.
T-1 Up-line 6+6 hold down DL= -6000 lbs LL= +4800 lbs TL=-1200 lbs ‹10
T-1 Dn-line 6+6 hold down DL= -6000 lbs LL= -6600 lbs TL= -12600 lbs* ‹10

*Note to construction crew and lift mechanics: STRUCTURAL DAMAGE will occur if haul rope derails and misses cable catchers on up-line assemblies when lift is in a loaded condition. This will impart an unbalanced load that will exceed tower design. FAILURE WILL OCCUR if lift is started. We recommend a counter weight with lifting eye under each side of this tower to facilitate re-railing.

T-2 Up-line 4support DL=+1000 lbs LL=+4000 lbs TL=+5000 lbs ‹2
T-2 Dn-line 4 support DL=+1000 lbs LL=+4000 lbs TL=+5000 lbs* ‹2

*Note to construction crew and lift mechanics: down line TL will only occur if carriers are loaded in both spans.