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Slider Joint

In one sentence: Allow a part to move linearly along one axis while locking all rotation and all off-axis translation — the joint for pistons, drawers, and linear rails.

Overview

Workbench: Assembly
Menu: Assembly → Joints → Slider
Shortcut: S
DOF removed: 5 | DOF remaining: 1 (translation along one axis)

A Slider joint defines a slide direction and allows one part to translate freely along it while all other motion is locked. The selected geometry defines the slide axis or direction.

Intuition

Think of a drawer in a cabinet. It slides in and out along one axis — forward and backward. It cannot rotate, cannot move sideways, cannot tip. A Slider joint enforces exactly this constraint.

When to use it

  • Piston inside a cylinder bore.
  • Dovetail slide or T-slot rail carriage.
  • Any part that moves in a straight line relative to another with no rotation.
  • Rack in a Rack and Pinion coupled joint — the rack must have a Slider joint first.

When NOT to use it

  • Don't use Slider if the part also rotates — use Cylindrical (independent rotation + translation) or Screw (linked rotation + translation).
  • Don't use Slider for curved paths — Assembly joints only support straight-line translation. For curved paths, use a scripted simulation or multi-joint workaround.

Step-by-step

  1. Press S or choose Assembly → Joints → Slider.
  2. Click a cylindrical face, linear edge, or planar face on Part 1 — this defines the slide direction (a cylindrical face uses its axis; a linear edge uses its direction; a planar face uses its normal).
  3. Click the corresponding feature on Part 2.
  4. Optionally set an Offset (initial position along the axis) in the task panel.
  5. Click OK.

Parameters

Parameter Type Default Description
Reference 1 Link — Axis-defining geometry on Part 1
Reference 2 Link — Axis-defining geometry on Part 2
Offset Length 0 mm Initial position of Part 2 along the slide axis

Distance limits

After creation, the joint's Data properties include optional travel limits:

Property Description
EnableLimitLength Toggle to enable min/max travel limits
LimitLengthMin Minimum travel position (mm)
LimitLengthMax Maximum travel position (mm)

Python API

Minimal example

import FreeCAD as App

doc = App.ActiveDocument
asm = doc.getObject("Assembly")

joint = asm.Joints.newObject("Assembly::JointObject", "Piston")
joint.JointType = "Slider"
joint.Reference1 = (doc.getObject("Cylinder"), ["Face3"])
joint.Reference2 = (doc.getObject("Piston"), ["Face1"])
doc.recompute()

Common mistakes and pitfalls

Selecting a planar face gives translation along the normal, not in the plane

When you select a planar face, the slide direction is the face's normal — perpendicular to the face. If you want the part to slide along the face, select a linear edge that runs in the desired direction instead.

Slider + Revolute on the same axis = Cylindrical

A Slider and a Revolute joint between the same two parts, both on the same axis, is equivalent to a single Cylindrical joint. Using all three would be over-constrained. Choose the single joint type that matches the intended motion.

Rack must have a Slider before adding Rack and Pinion

The Rack and Pinion coupled joint requires the rack to already have a Slider joint. The coupled joint adds the ratio link on top of the existing Slider — it does not create the slider constraint.

See also