how many stages should my slide have

You should use either a two-stage or a three-stage slide system, selecting the lowest number of stages that can achieve your required reach. This lesson does not state a single universal number of stages, as the choice depends on your specific reach requirements, rigging style, and load capacity.

Each additional stage you add increases the complexity of your system because linear slides stack tolerances, accumulate friction, and multiply forces. If you use a cascading rigging, adding more stages (N) multiplies your extension speed by N but divides your lifting force by N; for instance, a three-stage cascade is three times faster but only lifts one-third as hard as a single-stage equivalent.

Furthermore, more stages allow for longer extension, but a fully extended slide acts as a cantilever where deflection scales with the cube of the length. This creates droop that increases friction and can stall your motor under load.

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Cascade Versus Continuous

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The rigging determines the trade you are making.

Cascading. Each stage pulls the next, so with N moving stages the top stage moves N times as fast as the string is taken up. Speed is multiplied by N, and the available force is divided by N.

Continuous. All stages move together at the same rate, driven by a single continuous loop. No multiplication in either direction.

This is the trade teams forget. A three stage cascading slide extends three times as fast and lifts one third as hard as the same spool and motor in a continuous rigging. Sizing the motor for the extension speed and forgetting the force division produces a slide that cannot lift its load.

If the cycle time requirement is the binding constraint and the load is light, cascade. If the load is heavy and the speed is adequate, use continuous rigging or add reduction. Do not choose the rigging by what is easier to build, since the two have very different force characteristics.

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Fill-in-the-Blank Practice

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A cascading slide with three moving stages extends three times as fast and lifts __________ as hard as a continuous rigging with the same spool.
As string wraps onto a spool, the effective spool diameter __________, so the slide speeds up and weakens as it extends.
A slide should be extended by hand with the string disconnected to check for __________ across the full travel.

Show answers

one third
grows (increases)
bind (friction, stiffness)

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Overview

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Lesson 8.2: Sizing Linear Slides, Spools, and Extension Speed

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Linear slides give reach far beyond the robot's frame, which is why almost every high scoring FTC robot has them. They also stack tolerances, accumulate friction, and multiply forces in ways that catch teams out.

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Deflection at Full Extension

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A slide at full extension is a cantilever, and Lesson 3.3 applies directly: deflection scales with the cube of the length. The tip of a fully extended slide with a load on it can droop noticeably, and that droop changes where the end effector actually is.

Two consequences: the scoring geometry must tolerate the droop, or be measured with the droop included; and the droop loads the stages in a way that increases friction, which is why a slide that moves freely unloaded can stall loaded at full extension.

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Once the slide is sized, the code drives it to positions and needs a hard limit it can trust.

See [Lesson 11.1: Touch Sensors](/docs/unit-11/touch-sensor).

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Try It

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Slide friction is a function of build quality more than of design, and it is usually the difference between a slide that works and one that does not.

Sources:

Non-parallel rails. Rails that converge or diverge along their length bind at one end of travel.
Twist. Rails that are square at the base and rotated at the tip load the bearings sideways.
Overtightened rollers. Preload that removes all play also adds substantial drag.
String angle. String that pulls at an angle rather than straight along the travel puts a side load on every stage.
Debris. Field tile dust in the bearing track.

The diagnostic: extend the slide by hand with the string disconnected. It should move smoothly under light finger pressure across the full travel. Anywhere it gets stiff is a bind, and fixing it recovers motor output you would otherwise buy with a larger motor.

Testing an unloaded slide on a bench is not the same as testing it loaded and cantilevered, where the deflection is largest. Test at the extreme of travel with the real load on it.

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Overview

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Lesson 8.5: Challenge: Build a Two-Way Linear Slide with Software Limits

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Linear slides are prone to mechanical failure if driven past their physical limits, either bottoming out at the base or over-extending at the top. Using a DigitalChannel touch sensor as a software limiter prevents the motor from applying power in a direction that would cause hardware damage, protecting the motor from stall current and the mechanism from structural failure.

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II. Review the Entry

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Below is a notebook entry from a team. Identify the 2 problems and rewrite the affected parts.

Date: 3 Nov        Subsystem: Slide

Today we worked on the linear slide. We tried a few things and
made some good progress. The new version is a lot better than
the old one and extends much faster now. It seemed pretty
reliable when we ran it. We will keep working on it next time.

Show answers

Problem 1 - No stated problem and no reproducible description of the change. "We tried a few things" cannot be reproduced, compared, or learned from. A reader cannot tell what was changed, so they cannot tell what caused the improvement, and if the slide regresses later nobody can revert to a known state.

PROBLEM
Slide took 3.1 s to reach full extension; the cycle time
requirement allows 2.0 s.

WHAT WE TRIED
Replaced the 1.0 in spool with a 1.4 in spool, keeping the
same 312 RPM motor and 3 stage cascade rigging.

Problem 2 - Results are adjectives, with no numbers and no trial count. "A lot better," "much faster," and "pretty reliable" are opinions. They cannot be compared to the requirement, cannot be compared to the previous version, and give no basis for deciding whether the mechanism is competition ready.

RESULT
Extension time 2.2 s, averaged over 10 runs (range 2.1 to 2.4 s).
Full extension reached 10/10. Two runs showed the third stage
binding briefly near the top of travel.

NEXT
Still 0.2 s over the requirement. Check whether the bind at the
top of travel is costing time; inspect third stage alignment
before changing the spool again.

On the closing line: "We will keep working on it next time" is not a Next step. A Next step names the specific question the following session will answer.

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Arms > Arms for Extension > Pivot Extension

Game Manual 0·CC BY-NC 4.0

A pivot extension mechanism is an arm with a linear extension (typically linear slides) attached. This allows the linear extension to provide the desired extension length for both horizontal and vertical extension along with removing the need for a transfer stage.

(Figure: goBILDA Robot in 3 Days, Into The Deep, pivoting linear slides)

(Figure: 19098 Eastern Foxes, Into The Deep, Winning Alliance First Pick (Franklin), pivoting linear slides)

(Figure: 8417 'Lectric Legends, Rover Ruckus, Finalist Alliance First Pick (Ochoa), single arm + custom belt driven linear extension)

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Drawer Slides

Game Manual 0·CC BY-NC 4.0

You've definitely used a drawer slide before - at least two of them are mounted to almost any drawer that you've opened. Teams use these drawer slides for linear motion, often stacking them using 3D printed spacers to achieve plenty of extension.

These slides are available from a number of different vendors, and come in many varieties, so choosing the right slide can seem overwhelming. Steel drawer slides are common, but can be hard to mount, as they aren't made to be stacked. Aluminum drawer slides, such as the MiSUMI slides, are generally the best option for teams.

We recommend the Viper Slides or MiSUMI slides for newer teams. Viper Slides are cheaper and interface better with goBILDA products, while MiSUMI slides interface better with REV products. Viper slides are also available in a complete kit (https://www.gobilda.com/2-stage-viper-slide-kit/)

Note: When running two sets of drawer slides together, they should be mounted to face each other in order to reduce sag. This is especially important for horizontal extensions.

(Figure: 7236 Recharged Green, Rover Ruckus)

On 7236's robot, the slides are not facing each other. The sag caused by this is largely mitigated by supporting the extension at the end with omni wheels.

(Figure: 12791 Iterative Intentions, Power Play)

On 12791's robot, the slides face each other, which decreases sag in comparison to other orientations.

Listed below are the recommended drawer slides.

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Linear Bearing Slides > MGN Series Linear Bearing Blocks

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MGN linear bearing blocks are the most common linear bearing blocks in FTC. They are the smallest of a larger classification of slides made by HIWIN Corporation. They come in several sizes, the smallest being MGN7 and the largest being MGN15.

It is recommended that teams only use MGN7, MGN9, and MGN12 slides. A pair of MGN12 slides is sufficient to lift a 40lb (20kg) robot. MGN9 slides are similar in weight and scale to MiSUMI drawer slides. MGN7 slides are smaller, but they are slightly more inconvenient to work with because they use smaller M2 screws for mounting.

If ordered directly through industrial supply centers such as HIWIN, these slides can be expensive. For hobbyists, however, they are sold at more reasonable prices through sellers on Amazon and eBay, although these sellers may have questionable quality control. You can find more information with a Google search of "improving cheap MGN slides"

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Annotated Code

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package org.firstinspires.ftc.teamcode;

@TeleOp(name="Safe_Slide_Demo")
public class SafeSlideDemo extends OpMode {

    private DcMotor liftMotor;
    private DigitalChannel limitSwitch;

    @Override
    public void init() {
        liftMotor   = hardwareMap.get(DcMotor.class, "lift_motor");
        limitSwitch = hardwareMap.get(DigitalChannel.class, "limit_bottom");

        limitSwitch.setMode(DigitalChannel.Mode.INPUT);
        liftMotor.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);
    }

    @Override
    public void loop() {
        double power = -gamepad1.left_stick_y;

        // Active-low: false = pressed, so invert for intuitive logic
        boolean isAtBottom = !limitSwitch.getState();

        // Block downward movement if the bottom limit is triggered
        if (isAtBottom && power < 0) {
            power = 0.0;
            telemetry.addData("Safety", "BOTTOM LIMIT REACHED");
        }

        liftMotor.setPower(power);
        telemetry.addData("Slide Power",   power);
        telemetry.addData("At Bottom",     isAtBottom);
        telemetry.update();
    }
}

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Design Tradeoffs > Single/multi axis arm Linear extension

Game Manual 0·CC BY-NC 4.0

Important: Teams are generally advised to stay away from arms and move in the direction of linear slides, primarily due to the issue of complexity. This is because arms typically are less effective than linear extensions and are harder to implement properly.

Arms require a high gear ratio, so arms must be supported extremely well to bear the torque that the motor provides. A poorly supported and/or constructed arm will cause the driver needless pain as it is exceedingly difficult to line up an arm that constantly shakes. In contrast, linear extensions do not need to worry about gear ratios and gearboxes. They can be optimized to be more efficient than arms, and typically are more precise, as linear motion is easier to control than angular motion. Another positive is that linear slides can have more extension than arms, with some reaching 5+ feet in length.

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Simulator Challenge

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Use the simulator below to practice protecting a lift with an active-low bottom limit switch and lesson-specific starter code.

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How to Keep a Linear Slide Safe in Both Directions

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This challenge requires combining concepts from Units 7 and 8. You must:

Map both a DcMotor and a DigitalChannel in init()
Read the touch sensor state each loop cycle
Apply conditional logic that overrides driver input when the sensor is triggered

A critical hardware nuance: the REV Touch Sensor uses active-low logic. getState() returns true when the sensor is not pressed and false when it is pressed. This means you must use the logical NOT operator (!) to invert the signal for intuitive use: boolean isPressed = !limitSwitch.getState().

The limiter logic then becomes: if the driver is commanding the slide downward (negative power) AND the bottom limit is pressed, override the power to 0.0. Movement upward is still permitted because the limiter only blocks the dangerous direction.

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Linear Bearing Slides

Game Manual 0·CC BY-NC 4.0

Linear bearings are specialty ball bearings. Unlike radial ball bearings, they run balls through cyclical tracks along the length of a rail or shaft to create smooth, stable, precise linear motion. They are used in many industrial automation systems, including 3D printers and CNC machines.

Attention: Unlike with most slides, the balls of linear bearings are not well retained when the bearings are removed from their rails. Linear bearings with a few balls missing have slightly decreased performance. Be sure to handle and store the bearings with care and to be prepared if their balls fall out.

There are two types of linear bearings generally used in FTC: round linear bearings and linear bearing blocks.