Different types of sheave assemblies.
Sheave assemblies are the wheels located on lift towers.  Towers are placed on a
lift line to support the cable and transfer the live load of passengers and chairs to
the ground.  Based on the calculated weight for each tower section, a different
number of sheaves are needed
If the weight is overall positive (based on our sign convention as positive going
down) a tension assembly will be needed.  This means that the cable wants to
travel in the downward direction.  

The second scenario is the compression assembly.  This is used when the load is
close to zero.  A reason why these towers are installed is that loads change based
on the amount of chairs that are loaded and it might be desired that the cable be in
a certain position no mattering the chair loading situation.

The third common assembly is the depression one.  These are used if the cable
wants to travel in a negative direction.  Towers like these are usually found right
before steep inclines in the cable line.
* Tension Assemblies:
* Depression Sheaves:
Compression Sheaves:
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