Why Your FTC Linear Slides Bind Halfway Up (and How to Rig Them)
Linear slide binding mid-extension is caused by diminishing slide overlap multiplying bearing friction, angled string lines causing racking, or tension loss in continuous rigging. Here is how to diagnose and fix it.
If your FIRST Tech Challenge (FTC) lift extends smoothly for the first ten centimetres and then abruptly seizes halfway up, your slides are not defective. You are experiencing one of two predictable mechanical failures: diminishing contact overlap multiplying normal forces on your bearings, or cable routing that pulls the carriage sideways instead of parallel to the slide path.
Linear slides operate under severe cantilever loads during match play. When an intake or claw extends outward, gravity creates a rotational torque (a moment) across the slide assembly. As each stage extends, the physical overlap between adjacent slide segments decreases. Because mechanical moment is force multiplied by distance, halving the overlap distance doubles the normal force on your end bearings. If your rigging adds even a small lateral or off-axis vector, friction overtakes motor torque, and the lift binds.
Continuous vs. Cascaded Rigging: The Friction Arithmetic
How you route your lift cord determines how motor torque is distributed and how friction accumulates across stages. Most FTC teams build either continuous or cascaded lifts using off-the-shelf slides (such as goBILDA Viper slides, REV ultra-planar slides, or SAR3310 ball-bearing rails).
| Rigging Method | Line Speed vs. Motor | Motor Load | Stage Movement Order | Primary Failure Mode |
|---|---|---|---|---|
| Continuous | Fast (1:1 with spool) | Low torque demand ($1 imes$ lift weight) | Unpredictable (lowest-friction stage moves first) | Cable slack, pulley de-tracking, mid-stroke racking |
| Cascaded | Multiplied ($N imes$ spool speed) | High torque demand ($N imes$ lift weight) | Simultaneous (all stages move together) | Line stretch, stage timing mismatch, high motor stall risk |
In a continuous lift, a single unbroken cable runs from a spool, over pulleys at the top and bottom of each intermediate stage, and anchors to the final carriage. While easy to build with stock hardware, continuous rigging suffers from unpredictable stage order. The stage with the least resistance moves first. When that stage reaches full extension, the next stage abruptly takes the load. This sudden shift in centre of gravity frequently racks the intermediate stage, jamming the bearings against the rail.
In a cascaded lift, each stage has an independent cable anchored to the stage below it. When Stage 1 extends 100 mm, Stage 2 is forced to extend 100 mm simultaneously. Cascaded lifts prevent sudden weight shifts because all stages extend at a uniform rate, but they demand significantly higher torque from your gearbox and require precise tensioning per stage.
Why Binding Happens at the Midpoint
Mid-stroke binding almost always traces back to three physical causes:
- Non-parallel cable pull: If your lift string angles inward or outward even 3 to 5 degrees between the pulley and the stage anchor point, cable tension exerts a sideways force on the carriage. At rest, bearing preload resists this force. At half extension, where rail overlap is reduced, that sideways force cocks the carriage sideways in its track (known as racking).
- Friction multiplication through fixed bushings: Every 90-degree bend over a static standoff or low-quality plastic bushing incurs friction ($F_{ ext{out}} = F_{ ext{in}} e^{ heta imes ext{friction coefficient}}$). By the time continuous string wraps through four intermediate stages, over half the motor power is lost to routing friction rather than lifting mass.
- Cable stacking on the spool: When UHMWPE cord (such as REV cord or generic Dyneema/Spectra) wraps over itself irregularly on a bare motor shaft, its effective spool diameter changes dynamically. This causes unequal line feed between the extension string and the retraction string, creating slack or severe over-tension mid-travel.
How to Rig Slides Correctly Without Custom Machining
You do not need custom CNC plates or exotic components to eliminate slide binding. Applying rigorous benchtop assembly principles solves almost all lift failures.
1. Keep String Lines Strictly Parallel
Every segment of string running between a pulley and an anchor point must run dead parallel to the slide extrusion in both the X and Y axes. Use spacers and shims beneath your pulleys to ensure the line leaves the pulley groove at the exact plane of the stage anchor screw. A string pulling even slightly off-axis acts as an efficient brake.
2. Eliminate String Slack with Dynamic Tensioning
Continuous lifts require a return (down-haul) cable to pull the slide down reliably. Because string stretches under load and spools rarely wrap symmetrically, rigid rigging will either slacken or bind. Terminate your return cable onto a heavy-duty tensioning spring or short loop of high-modulus surgical tubing anchored to the carriage. The spring absorbs spool diameter fluctuations while maintaining positive cable tension on all pulleys.
3. Standardise on Ball-Bearing Pulleys
Never route lift string over standoffs, aluminium spacers, or 3D-printed channels without bearings. Use dedicated ball-bearing pulleys or small flanged bearings (such as MR105 or F688 series bearings sandwiched between washers). The difference in rolling friction between a cheap flanged bearing and a static spacer is roughly an order of magnitude.
4. Use Dry Lubricants Only
Do not spray WD-40, silicone wet spray, or petroleum grease onto FTC slide tracks. In a competition pit or classroom floor, wet lubricants attract dust, plastic shavings, and foam debris from field elements, forming a grinding paste that destroys ball tracks and nylon sliders. Clean slide extrusions with isopropyl alcohol and apply a dry PTFE spray (such as dry Teflon lubricant), allowing it to dry completely to a matte film before reassembly.
5. Constrain Spool Winding
Avoid single-point winding on an open hex shaft. Use grooved spools or dual-drum winches where the extension string unwinds at the exact rate the retraction string winds up. If using standard 3D-printed spools, print dividing walls to separate the lift and return cords, preventing the lines from crossing over each other under load.
If you are re-evaluating your drivetrain and lift geometry for the upcoming season, explore our tested mechanical layouts and kits in our FTC robotics section.
