Load Revolution: The Gear Ratio, Not Leadscrew Pitch

A positioner translates motor-shaft rotation into load motion through two separate definitions: a mechanical transmission ratio and a load-unit scale. Mixing those definitions makes the displayed position wrong even when the motor moves correctly. Motor poles do not belong in the mechanical ratio.

Mechanical quantities and relationships

Motor revolution means one physical revolution of the motor shaft. The motor-revolution value represents the motor side of the transmission ratio. Do not multiply it by the number of motor poles. Pole count affects the relationship between electrical frequency and shaft speed; it does not change how many mechanical turns the shaft makes.

Load revolution means one revolution of the load-side reference shaft defined for position scaling. For a leadscrew axis, the clearest reference is normally the screw shaft. For a rotating indexer, the reference may instead be one index interval. Under that convention, one index interval can be displayed as 360 degrees, so a four-position table requires four displayed revolutions for one physical table turn.

Leadscrew pitch is the linear distance traveled by the nut during one physical screw revolution. It converts load-side rotation into linear travel; it is not the motor-to-load transmission ratio.

Let Nmotor be motor revolutions and Nload be load revolutions. The mechanical ratio is:

motor turns per load turn = Nmotor / Nload

Both configured revolution values must be integers. Equivalent integer pairs describe the same mechanics because multiplying both sides by the same number does not change the ratio.

Check 1 — Load-axis definition

Identify the physical shaft represented by one load revolution before entering P2506 or an associated load-unit scale.

1. Mark the intended load reference shaft.
2. Rotate that shaft through one physical turn without relying on the drive display.
3. Measure the resulting machine motion.

Expected reading for a leadscrew: one screw turn moves the nut by exactly one leadscrew pitch. If that reading is correct, define one load revolution as one screw revolution unless the machine coordinate convention requires a different reference.

Alternative reading: if one revolution is defined as an index position or another virtual machine interval, document that convention. The displayed revolution then describes machine coordinates rather than one complete physical turn of the final mechanism.

Setting one defined rotation to 1000 LU establishes resolution: one load revolution equals 1000 load units. It does not by itself require the load-revolution ratio value to be 1. The mechanical ratio still depends on how many motor turns produce the defined load motion.

Check 2 — Transmission-ratio reading

Disconnect the scaling question from the gearing question. Count motor-shaft turns and load-reference turns over the same movement.

1. Choose a movement large enough to count both shafts accurately.
2. Record physical motor turns as Nmotor.
3. Record physical load turns as Nload.
4. Convert fractional counts into an equivalent integer pair.

Expected reading for direct drive: one motor turn produces one screw turn, giving 1:1.

Expected reading for the stated geared example: 13.5 motor turns produce 3 load turns. Because the fields require integers, multiply both values by 2 and enter the equivalent pair 27:6. The pair 54:12 represents the same ratio. Reducing the ratio gives 9:2; use an equivalent pair accepted by the configuration fields.

27 / 6 = 54 / 12 = 9 / 2 = 4.5 motor turns per load turn

If the observed load turns are the reciprocal of the prediction, the motor and load entries have been exchanged. A reversed travel direction is a separate sign or direction problem; changing ratio magnitude is the wrong correction for direction.

Check 3 — Load-unit and pitch scaling

After the ratio matches the mechanics, define how many LU represent one load revolution. For a leadscrew whose load reference is the screw, assign the units for one pitch of linear travel.

Let L be load units per load revolution. The commanded scale per motor turn is:

LU per motor turn = L × Nload / Nmotor

If one screw revolution is 1000 LU and the transmission is 27:6:

LU per motor turn = 1000 × 6 / 27
                  = 222.222... LU per motor turn

This result is derived from the ratio; it does not mean a fractional revolution value should be entered. The integer revolution pair remains 27 and 6.

When position is expressed in physical distance, let P be the leadscrew pitch and R be load units per unit of distance:

LU per screw turn = P × R
linear travel per motor turn = P × Nload / Nmotor

Read the actual pitch from the screw documentation or measure nut travel over several turns. Do not substitute screw diameter, thread count without unit conversion, or motor encoder resolution for pitch.

Resolving configuration procedure

1. Define the load reference: one physical screw turn for a conventional linear axis, or a documented virtual interval for a rotating indexer.
2. Count corresponding motor and load turns. Express the result as an integer pair without changing the ratio.
3. Enter the motor-revolution value on the motor side and the load-revolution value on the load side. Keep motor pole count out of both fields.
4. Assign the required LU to one defined load revolution. If one screw turn must equal 1000 LU, enter that scale independently of the gear-ratio pair.
5. For linear coordinates, relate one screw revolution to the measured leadscrew pitch and the chosen distance-unit resolution.
6. Apply the configuration, then perform the low-speed verification checks below before releasing normal motion.

Chapter 6.2 of the Siemens G120 Basic Positioner Function Manual describes defining the LU resolution of one rotation. Its STARTDRIVE illustrations use the same parameter definitions referenced for this mechanics setup.

Verification readings and recurring errors

1. Check 1: motor ratio. Command one defined load revolution at low speed. Expect the motor shaft to turn Nmotor/Nload revolutions. A reciprocal result means the ratio entries are reversed.
2. Check 2: load rotation. For a screw-referenced axis, expect one displayed load revolution to produce one physical screw turn. A different count means the load reference or gear ratio is wrong.
3. Check 3: linear displacement. Measure nut travel for one screw turn. Expect one leadscrew pitch. Correct rotation with incorrect travel points to a wrong pitch value or the wrong mechanical screw specification.
4. Check 4: load-unit display. If one load revolution is configured as 1000 LU, expect a position change of exactly 1000 LU for the defined load revolution. Correct mechanics with the wrong display points to load-unit scaling.
5. Check 5: repeat movement. Command several identical moves in both directions. Expect the same magnitude each time. A constant scale error indicates configuration; direction-dependent deviation points to backlash, coupling slip, or other mechanical lost motion.

Frequently asked questions