Linear slides and lifts

Robots start inside an 18-inch cube, and most games put the scoring positions further away or higher up than that. Game Manual 0 notes that more than 18 inches of reach is needed in nearly every game and more than 24 inches is preferred. Linear slides are the most common way to get it: stacked rails that telescope out, pulled by string, belt or chain.

Slides and stages

Stage

One moving section of a slide. A "three-stage" slide has a fixed base plus moving sections that each slide out of the one below.

Common types in FTC:

• Drawer-style slides — ball-bearing slides like the ones in a kitchen drawer, made stronger for robots. goBILDA's Viper-Slides are an example, sold as kits with rigging included.
• Extrusion slides — built from extrusion with bearings riding in the slots, such as REV's 15 mm linear motion kit. Flexible, but they need careful setup to run smoothly.
• Linear bearing slides — rods or rails with linear bearings, usually custom.

Most teams mount two slide sets side by side with a carriage between them, so the load does not twist a single slide.

Rigging: how the string makes it extend

Rigging is the path the string (or belt) takes through the stages. A motor-driven spool winds string in to extend; a second string, wound the opposite way on the same shaft, pulls it back in. There are two main patterns.

Continuous rigging

One extension string runs from the spool, up over the top of the base stage, down to the bottom of the first stage, up over its top, and so on, ending anchored to the last stage.

• The last stage moves first. Once it hits its stop, the next one moves, and so on.
• The slide extends as fast as the spool reels in string — 1:1.
• The spool pulls with roughly the weight of what is moving.

Game Manual 0 notes that continuous rigging can usually run on a fairly low gear ratio.

Cascade rigging

Each stage has its own string, anchored so that moving one stage drags the next one with it.

• All stages move at once. With three stages, the top moves three times as fast as the first stage.
• The spool has to pull roughly three times as hard for a load at the top. Speed and force trade off exactly like gearing.
• More strings to keep tensioned — one per stage.

Game Manual 0 also points out that adding stages to a cascade means you need more reduction to keep the same speed.

Sizing the spool and motor

A spool is a pulley that winds string, and it acts exactly like a wheel. Its radius sets the trade between speed and force:

string force = motor torque / spool radius
string speed = spool RPM × π × spool diameter

A smaller spool gives more force and less speed, which is the same as adding reduction. Swapping the spool is often easier than swapping the gearbox.

Worked example: a continuous-rigged lift

Your slides, carriage and the game element together weigh about 1.5 kg. The spool is 38 mm in diameter (radius 0.019 m).

weight          = 1.5 kg × 9.81 m/s² ≈ 14.7 N
torque to hold  = 14.7 N × 0.019 m  ≈ 0.28 N·m
with 2× margin  ≈ 0.56 N·m

The margin covers friction in the slides, acceleration, and the battery sagging late in the match. Now compare it with goBILDA's listed stall torques:

The 435 RPM motor lifts comfortably, and at free speed it would cover 70 cm in under a second (a real lift is slower). The 1150 RPM motor is far faster on paper, but it would be working close to stall — hot, slow under load, and likely to stall if anything binds.

Now try a three-stage cascade with the same spool and the 435 RPM motor. If most of the weight is at the top, the spool has to pull about three times as hard:

torque with margin ≈ 3 × 0.56 ≈ 1.68 N·m  →  ≈ 92% of the 435's stall torque

That will not work as it stands. You would need a smaller spool, more reduction, or a stronger motor — and in exchange, the top of the slide moves three times as fast.

Rigging that stays rigged

• Use low-stretch braided line, not sewing thread or fishing line that stretches.
• Never let string wrap over itself on the spool. Each overlapping layer makes the spool effectively bigger, which changes speed, force and tension. Make the spool wide enough for all the string in one layer.
• Make the extension and retraction spools the same diameter on continuous rigging, wound in opposite directions.
• Add a spring tensioner on each string run. String always loosens over time; Game Manual 0 strongly encourages at least one per slide set.
• Keep both sides the same. Drive both slide sets from one shaft and tension them equally, or one side lags and the carriage twists and jams.
• Manage the wires. Anything on the carriage needs cable that can extend with it — a cable carrier (drag chain) or coiled cord — so it cannot snag.

Gravity, encoders and limits

A vertical lift is always being pulled down. Three things keep it under control:

1. Holding position. With the motor off, the load can back-drive the lift down. Setting the motor to brake when power is zero helps, but heavier lifts usually need code that holds position — see PID control in subsystems.

2. Knowing where it is. The motor's encoder counts rotations of the spool. For the 435 RPM motor, goBILDA lists 384.5 counts per output-shaft revolution. With a 38 mm spool on continuous rigging:

string per revolution = π × 38 mm ≈ 119.4 mm
counts per mm         = 384.5 / 119.4 ≈ 3.22
600 mm of travel      ≈ 1,930 counts

Encoders forget their position when the robot powers off, so the lift needs to find a known zero each time — usually with a limit switch at the bottom. See homing mechanisms.

3. Stopping at the ends. Every slide needs mechanical hard stops so a stage cannot be pulled off the end. Then add soft limits in code — a maximum and minimum encoder count — so the motor slows down before the hard stop instead of slamming into it at full power.

Next

Slides move in straight lines. Arms move in arcs — and fight gravity very differently along the way.

Sources and further reading

• Game Manual 0 — Linear Motion Guide
• Game Manual 0 — Rigging (continuous, cascade, retraction, spool sizing, tensioners, cable management)
• Game Manual 0 — Extrusion Slides
• goBILDA — 5203 Yellow Jacket 435 RPM motor (stall torque and encoder resolution)
• goBILDA — Yellow Jacket planetary gear motors
• FIRST Tech Challenge Competition Manual — starting-size and extension rules for this season