Cascade Lift
Line tension, how much of it is friction rather than weight, and whether the geometry lets the stages slide at all.
Inputs
The stages
Geometry
How much of each stage stays inside the one below at full extension.
The lever that presses the stages into their guides. Usually the biggest thing you control.
How far apart the bearings holding that lever are.
Sliding
Metal on metal is around 0.25. Rollers or bearings get you near 0.05.
Used to work out how much the spool grows as line winds on top of itself.
Results
Will it move?
—
Line tension needed
—lbf
Of that, actual lifting
—lbf
Of that, friction
—lbf
Friction is
%
Binding margin
—×
Total travel
—in
Tension with rollers instead
—lbf
Spool grows by
—%
Watch the stages come out
Each stage carries the one above it and slides its own length out, less the overlap it has to keep. Run it out and count: the travel is one stage's stroke multiplied by the number of stages, which is why a cascade beats a four-bar for height and pays for it in the line tension at the bottom.
1.0 is the point where the stages jam
of the total the spool must pull
How this is calculated
A cascade rigs several sliding stages so one spool drives all of them at once. Each stage moves relative to the one below it, so the top travels further and faster than any single stage does.
travel = stages × (stage length − overlap)With your numbers
Most of the tension is not lifting
Add up what the spool pulls against and the weight is the smaller half. In the build this page opens with, 55% of the line tension is friction, not load. A cascade is not hard to lift. It is hard to slide.
The reason it gets so bad is that friction is not just added on, it is multiplied. Every stage's friction is felt by every run of line below it, so a stage near the top has its drag counted several times over. Adding a fourth stage costs far more than a third did.
The eccentricity is the thing you control
The load rarely sits on the slide axis. It hangs out in front, and that offset levers each stage into its own guides. The harder it levers, the harder everything rubs.
binding margin = bearing contact ÷ (2 × friction × offset)Below 1, the stage wedges itself into its guides hard enough that it cannot slide at all. The lift does not lift slowly — it locks. And because it is a geometry failure rather than a power one, a bigger motor makes no difference whatsoever. Halve the offset, or lengthen the bearing contact, or reduce the friction.
Rollers
Sliding metal on metal is roughly 0.25. Rollers or bearing blocks get near 0.05, and because the friction term is multiplied through every run, the effect on tension is out of proportion to the change: for the default build it cuts the required tension by 44%.
That is usually a better investment than more motor, because it also raises the binding margin — which more motor does not.
The spool grows as it winds
Line does not lie in one neat layer. Once a layer fills, the next winds on top, and every turn after that pulls more line per revolution than the last. So the same motor rotation moves the lift further at the top than at the bottom.
This is why a cascade tuned by encoder counts at the bottom overshoots at the top. The figure here is how much the effective radius grows over the full travel; anything above a few percent is worth handling in code rather than trimming out by trial.
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Sources & assumptions
No VEX data. This is statics and friction, and applies to any cascading slide.
Checked against independently derived values. The repository carries a verification script that reproduces the per-run tensions, confirms the per-stage force balance identity at every stage, and derives the spool growth by integrating layer area rather than assuming the formula. It agrees on the reference build: 7.9 lbf of ideal tension, 17.57 total, 55% friction share, and a 44% reduction with rollers.
Where this goes next
Explore further on the full mapWhere the answers on this page can be used, and which tools could have given you the numbers it asks for.
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Cascade Lift
Where your answer goes
Required Lift Torqueturn that line tension into the motor torque needed at the spooluses your lift load againTipping & Wheelie Thresholdsee how tippy the robot gets with the load right at the topbrings your lift height with itShaft Deflectionsee how much the spool shaft bends with the line pulling on ituses your lift load againBearing Load & Support Placementsee how hard the line load presses on each bearinguses your lift load againa number carries across the next question it raises