Lift Systems
Guide

How It Works

A direct arm rotates around a single fixed pivot. Motors drive the pivot directly or through a gear reduction. The end of the arm travels an arc — which means the end effector (claw, intake, cup) also rotates as the arm rises. This is fine for dumping or launching, but not for placing a piece at a precise angle.

When to Use It

• Scoring height requires less than a 90–120° swing from floor to target
• The end effector does not need to remain level (dumper, catapult, claw that releases)
• You have limited motor budget and need the simplest possible mechanism
• Speed matters more than precision — direct arms move fast because they are light

Gear Ratio for Arm Motors

Torque requirement increases with arm length and load. A long arm with a heavy game piece at the end requires significant gear reduction. Starting point: calculate the torque at the pivot (weight × distance from pivot) and compare to your motor’s stall torque at your chosen reduction. Always include a 30–40% safety margin — motors should not stall under normal load.

Arm Presets in Code

Use position presets driven by encoder counts rather than timing. Timing-based arm moves drift as batteries discharge. Encoder-based presets are repeatable across a match. Set presets for: floor pickup, carry position, scoring height, and safe travel height. See the Full Competition Code guide for implementation.

How It Works

A four-bar has exactly four rigid links connected at four pivot points: the robot chassis (ground link), the driven arm, the output link (carries the end effector), and a coupler connecting them. When the driven arm rotates, the output link maintains its angle relative to ground — meaning whatever is attached to it stays level. This is called a parallel four-bar when the two side links are equal length and parallel.

Key Design Rules

• Equal-length parallel links. For a true fixed-orientation output, the two side bars must be the same length and the pivot points must form a parallelogram. Measure in CAD, not by eye.
• No slop in pivot joints. Each of the four pivots adds angular error. With 0.5° of slop at each pivot, the output can drift 2° — which is visible at full extension. Use proper shoulder screws or bearing flats at every pivot.
• Counterweight or rubber band assist. Four-bars have a heavy moment arm at full extension. Adding elastic to assist the upward motion reduces motor load and allows a smaller motor reduction while still holding position.
• CAD the full range of motion. Before cutting metal, animate the four-bar in Onshape through its full travel. Check for interference with the robot body, wheels, or electronics at every position.

Six-Bar Preview

A six-bar is a four-bar with a second stage added — the output link of the first four-bar becomes the ground link of a second. This gives more reach (higher scoring height) at the cost of more pivot points, more slop, and more weight. Use a six-bar only if a four-bar cannot reach the required height.

Double Reverse Four-Bar (DR4B)

The DR4B stacks two four-bar stages in opposite directions. The bottom stage lifts up; the second stage — which is reversed — lifts further while keeping the end effector absolutely level at all heights. This is the mechanism used in nearly every high-stack VRC game because no other configuration reaches the same height with the same end-effector stability at competitive weight.

DR4B Design Notes

• Every pivot must be tight. The DR4B has 8 pivots. Slop at any of them appears magnified at the top. Use bearing flats or precision shoulder screws throughout.
• Rubber band staging. Band the first stage to assist its portion of the weight, then band the second stage for its load. Do not use a single set of bands for the whole system — the leverage ratio changes at every position.
• Rigid chassis attachment. The DR4B’s ground link must be bolted rigidly to the robot frame with at least two attachment points. A single-bolt pivot flexes and destroys the geometry.
• Motor position matters. Driving the bottom stage directly is most common. Some teams add a second motor to the top stage for faster cycle time — verify your motor budget allows this.
• Starting height check. The DR4B at rest must fit within the robot size limit. Design the folded position in CAD and measure — do not assume it fits.

High-Cycle Failure Points

• Pivot bearings wear loose. After hundreds of cycles, bearing flats can spin in their holes. Check by hand — any play in a pivot bearing means it needs to be tightened or replaced. Check pivots after every competition day.
• Elastic stretches and loses assist force. Rubber bands fatigue. Mark your rubber band configuration and check tension before each match. Carry spares in the pit box with the same known count and stretch.
• Shaft collars migrate inward. Collars on pivot shafts can walk under load. Loctite your shaft collar set screws and check at each pit stop.
• Motor overtemperature on long cycles. A lift that is up for long periods with the motor holding position generates heat. Add a hold current limit in code — switch to a lower holding power once the target position is reached.

Pre-Match Lift Check

• Manually move the lift through its full range — no binding, no grinding, smooth through the whole travel
• Check all pivot screws — none backing out, all seated
• Test elastic tension — same tension as after last successful match
• Run 3 full cycles and verify position preset accuracy

Notebook Evidence

• CAD screenshot of lift at full extension with height dimension labeled
• Comparison table: Direct arm vs four-bar vs six-bar vs DR4B with pros/cons for your specific game application — this is your decision matrix
• Gear ratio calculation showing torque at pivot vs motor output — shows you verified the system will not stall
• Cycle time data table from 20 repeat cycles — average and variance
• Preset accuracy data — measured position vs. target at each preset height