Skip to main content

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

Technical Context​

Linear slides extend a mechanism beyond the robot's frame. Each added stage also contributes tolerance, friction, and transmission load that must be included in the design.

Cascade Versus Continuous​

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.

Decide which one you are short of

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.

The Spool​

The spool converts motor rotation into string travel:

string_speed = pi * spool_diameter * spool_rpm
string_tension = motor_torque / spool_radius

A larger spool is faster and weaker. A smaller spool is slower and stronger. The trade-off behaves like a gear ratio, and the spool is often easy to change on an existing slide.

One detail teams miss: as string wraps onto the spool, the effective diameter grows. A spool with several wraps of thick string can be noticeably larger at the top of travel than at the bottom, which means the slide speeds up and weakens as it extends. Use thin high strength line and a wide spool so the wraps sit side by side rather than stacking.

Try It​

BringStage count, spool diameter, and the motor output after its gearbox.
ChangeThe rigging, and compare what cascading does to speed against force.
ReadLifting force against the load. Cascading divides force by the stage count.

Linear Slide Sizing

Cascading buys speed with force. Decide which one you actually need.

Scenarios
Cascading rigging
fixed basestage 1stage 2stage 34.0 lbspoolstring take-up x3 at the tip68.6 in/sextension speed7.5 lbflifting force28 intotal travel
StagesLoadString

Each stage adds its own travel, so the tip moves 3 times the string speed. The same factor divides the force, which is the trade teams forget when they size the motor for speed alone.

3xStage multiplierSpeed up, force down
68.6Extension speed (in/s)No load applied
0.41Time to full travel (s)
7.5Lifting force (lbf)At stall, after losses
1.88xSafety factorLoad is 4.0 lbf

Good margin. Check the descent too: with this much reduction the slide may not back-drive, which is often desirable for holding position but means a failed motor leaves the slide stuck up.

Efficiency on a slide is lower than on a gearbox: string friction, pulley losses, and any bind from misalignment all subtract. 75% is a reasonable starting estimate for a clean build and optimistic for a first prototype. Load weight should include the game element, the end effector, and everything the slide carries.

Values are not saved. Nothing is sent anywhere.Open in the workbench

Friction and Bind​

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.

A slide that binds at full extension will bind harder under load

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.

Retraction​

Extension is powered. Retraction is often not, or is powered only weakly, which produces two problems.

Gravity retraction is unreliable on a horizontal slide, since there is nothing pulling it back. Use a return spring, surgical tubing, or a second string in the opposite direction.

Slack string tangles. When a slide retracts faster than the spool takes up string, the string goes slack, comes off the spool, and wraps somewhere it should not. Keep the string under tension throughout the travel, with a tensioner or a spring loaded idler.

Deflection at Full Extension​

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.

The scoring geometry must either tolerate the droop or be measured with it included. Droop also loads the stages in a way that increases friction, so a slide that moves freely without a payload can stall when loaded at full extension.

Connects to the software track

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.

Photograph neededA linear slide fully extended and loaded, photographed from the side against a straight reference such as a level or a length of extrusion.Framing: Side elevation, straight reference held horizontal in frame, far enough back to see the whole extension.Add the file to static/ and pass its path as the src prop.
A slide at full extension with a load on it. The droop at the tip is what changes where the end effector actually is, and the same deflection is what makes the stages bind.

Fill-in-the-Blank Practice​

  1. A cascading slide with three moving stages extends three times as fast and lifts __________ as hard as a continuous rigging with the same spool.
  2. As string wraps onto a spool, the effective spool diameter __________, so the slide speeds up and weakens as it extends.
  3. A slide should be extended by hand with the string disconnected to check for __________ across the full travel.
Show answers
  1. one third
  2. grows (increases)
  3. bind (friction, stiffness)

Exercise​

Disconnect the string and extend your slide by hand through its full travel, noting where it stiffens. Then measure the actual extension time under load and compare against the calculator's prediction. The gap is your real rigging efficiency, and if it is far below 75% you have a bind worth finding.

Ready to move on?

Only mark complete if you genuinely understand the material. Your progress will be saved in this browser.

Stuck on this lesson?

Ask about anything on this page. It can see which lesson you have open and which part you are reading.