Joint Limits and Motion Links

Overview#

A bare revolute or slider joint moves forever. Real mechanisms do not. Two tools make joint motion realistic:

• Joint limits set the minimum and maximum travel (and an optional rest position) for a single joint.
• Motion links tie the motion of one joint to another so that moving one drives the other at a defined ratio. Autodesk describes the Motion Link tool as specifying "a relationship between the degrees of freedom of the selected joints", which is a position-to-position relationship rather than a time-based motion study.

Together they let you model hinges that stop at 110 degrees, drawers with defined travel, gear pairs, rack and pinion, cam-driven followers (approximately), and pulley systems. They do not simulate forces or dynamics; they are kinematic.

Where to find it#

Step by step: joint limits#

1. Create a revolute or slider joint (see Joints and Joint Types).
2. In the Joint dialog, switch to the Motion tab and find Joint Motion Limits (or right-click the joint later and choose Edit Motion Limits).
3. If the joint has more than one degree of motion, pick it in Motion.
4. Set Minimum (for example, 0 deg or 0 mm).
5. Set Maximum (for example, 110 deg or 150 mm).
6. Optionally set Rest, the resting position within the range of motion.
7. Click Preview Motion (animates only that degree of motion), and tick With All Limits to preview with every degree of motion and limit applied.
8. Click OK. Drag the component to test: it should stop at both ends.

Every limit has an associated parameter, so limits can be expression-based in a parametric design. Flip swaps the minimum and maximum values.

Understanding the reference position#

Note: not confirmed in Autodesk's documentation: how the zero of a limit is defined (for example, the X axis for a revolute). Autodesk only says joints use Motion Limits while assembly constraints use Offset Limits. If your limits appear shifted, check the joint origin axes and the Angle in Joint Alignment, or adjust the limits.

Accuracy caveat#

A limit entered by eye or from a rough measurement may be approximate; contact sets can stop motion where parts touch. For precision, calculate the limit from geometry (for example, rail length minus carriage length) or use parameters, since each limit has one (general guidance).

Step by step: motion links#

1. Create at least two joints with motion (revolute, slider, cylindrical).
2. Choose Assemble > Motion Link.
3. In the dialog, select the Joints to relate (two joints; the dialog lists each joint name with its available motion).
4. For each joint, choose the motion to relate (for a Cylindrical joint, its rotation or its slide).
5. Set the Value for each motion (the value type depends on the motion selected), which defines the ratio. For example, Joint 1 = 360 deg and Joint 2 = 360 deg gives 1:1; for a rack and pinion where one revolution moves the rack 94.25 mm, enter 360 deg for the pinion and 94.25 mm for the rack.
6. Use Reverse to reverse the direction of the motion if the second joint must move opposite to the first. External gear pairs rotate in opposite directions.
7. Click Animate to start or stop an animation of the relationship, then click OK. Drag one joint, or use Drive Joints; the linked joint follows.

Typical ratios#

Tip: Compute the link ratio using exact values, not rounded ones, to avoid drift over repeated rotations. Use the model's own parameters where possible.

Options reference#

Joint limits#

Motion link#

Motion links versus motion study#

Autodesk's Motion Study reference does not say how motion links behave during a study; test it on your own design.

Combining limits, links, and contacts#

• Put limits on the driver joint, not the follower, to avoid conflicts.
• If a linked follower has its own limit that contradicts the ratio, the driver can stop earlier than expected.
• Use Contact Sets to stop motion physically where parts touch, instead of guessing the limit value.
• Use contact sets for parts that must stop on contact; Autodesk does not document whether they work on two components that already share a joint, so rely on joint limits for those pairs.

Troubleshooting#

Tips and pitfalls#

Tip: Model gears with real teeth only if you need visible detail; a motion link provides the kinematics without meshing teeth.

Warning: A motion link does not prevent teeth from colliding or detect interference. Check clearance separately.

Tip: Name the joints "J_crank", "J_slider" so the link dialog is readable.

Warning: Limits on a joint whose origin is rotated by 90 degrees may not read as you expect. Show the joint origins to verify the axes.

Tip: Set a Rest position equal to the default state of the product (door closed, lever up), so Reset gives a predictable pose.

Warning: Links between more than two joints in a loop can make motion impossible. Keep link graphs as trees.

Worked example: a rack and pinion with real numbers#

A pinion with a pitch radius of 10 mm drives a rack. One full turn of the pinion moves the rack 2 x pi x 10 = 62.83 mm. You want a rack stroke of 200 mm.

1. Set the rack Slider limits to 0 and 200 mm.
2. The pinion then needs to turn 200 / 62.83 = 3.18 turns, which is 1146 degrees. Leave the pinion Revolute joint unlimited rather than limiting it to 360.
3. Create a Motion Link, select the pinion joint first and the rack joint second, and set the relationship so one rotation of the pinion corresponds to 62.83 mm of rack travel. Enter 360 deg for the pinion and 62.83 mm for the rack.
4. Drag the pinion. The rack should stop at 200 mm when the pinion reaches 1146 degrees. If the rack stops early, one of the limits is smaller than the link requires; if the pinion keeps turning with a stopped rack, the joint limits are fighting the link.

For a simple gear pair with a 20-tooth gear driving a 40-tooth gear, the ratio is 20 : 40 so the driven gear turns half as many degrees, in the opposite direction. Use a negative value or the Reverse option if the gears should counter-rotate; a wrong sign shows up as both gears spinning the same way.

Limits and Rest: three numbers, three meanings#

Keep Rest inside the Minimum-to-Maximum range (general guidance; Autodesk's offset-limit topic requires the rest value within the range for constraints). Keep Rest inside the range and equal to the pose you modeled in.

Failure modes and fixes#

Checking a limit with a measurement#

Rather than trusting a typed value, drive the joint to its maximum and measure the real distance with Inspect > Measure. A 150 mm slider that should end 5 mm shy of a wall is easy to verify. If the measured gap is 8 mm, adjust the limit by 3 mm and remeasure. This takes a minute and prevents shipping a mechanism with mis-set stops.

Design guidance#

• Put limits on the joint that is physically stopped, not on its neighbor.
• Use one input joint and let motion links drive the rest; two independent drivers on one mechanism create conflicts.
• For cam-like or nonlinear relationships, motion links (whose value pairs define a fixed ratio, per the Value fields) are not enough; model the cam geometry and let contact do the work.

Practice exercise#

1. Build a baseplate (grounded), two gear disks (20 mm and 40 mm pitch diameters), and a rack.
2. Add a Revolute joint for each gear to the base, and a Slider joint for the rack.
3. Create a Motion Link between the small gear and the large gear with ratio 360 : 180, reversed.
4. Create a second Motion Link between the large gear and the rack with 360 deg : (pi x pitch diameter) mm.
5. Add limits of 0 to 200 mm to the rack slider.
6. Drag the small gear and verify the rack moves and stops at 200 mm.

Expected result: rotating the first gear drives the second in reverse and the rack linearly, stopping at the limit.

Stretch challenge#

Build a worm gear stand-in: a worm on a Revolute joint, a wheel on a Revolute joint, and a Motion Link of 30:1 (30 turns of the worm per turn of the wheel). Limit the wheel to 0 to 90 degrees. Expected result: the worm must turn 30 x 90 / 360 = 7.5 turns to move the wheel 90 degrees, and Drive Joints on the worm to 2700 degrees (7.5 x 360) puts the wheel at its limit. If the wheel stops short, check the sign and the ratio direction; if it overshoots, the wheel limit was not saved.

Related#

• Joints and Joint Types
• Contact Sets
• Motion Study
• Capture Position