BIZOMAT MAKEOS · Mechanics
Traction Hoist Check (Counterweight, Rope Tension Ratio, Sheave Traction, Motor Power)
For a counterweighted lift or hoist driven by rope traction on a grooved sheave, will the ropes grip without slipping loaded and empty, and what motor power does it need?
Check of a counterweighted traction drive: the counterweight from the car mass and a share of the rated load, the rope tensions on each side with the car full and empty, increased for acceleration, their ratio, the traction the sheave can give from the groove friction and wrap (as the belt friction tool, with the groove angle), and the motor power from the largest out-of-balance load, speed and efficiency, against the motor fitted.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Car (platform) mass | kg | Mass of the car or platform, its frame and its share of the ropes. |
| Rated load | kg | Rated load. |
| Counterweight balance (share of rated load) | % | Share of the rated load the counterweight balances (40–50% is usual). |
| Acceleration or braking | m/s² | Largest acceleration or emergency braking deceleration. |
| Sheave | ||
| Rope-to-groove friction coefficient | Friction between the rope and the sheave groove. | |
| Wrap angle | ° | Angle of rope wrap on the sheave. |
| Groove included angle (0 = round seat) | ° | Included angle of a vee or undercut groove (0 for a round seat). |
| Drive | ||
| Rated speed | m/s | Rated rope speed. |
| Drive efficiency | % | Efficiency of the motor, gearing and sheave. |
| Motor power fitted | kW | Rated power of the motor. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Car (platform) mass
- What it is
- The mass of the empty car or platform with its frame, fittings and its share of the hoist ropes.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 1000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 600 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Include doors, sling and any fixed equipment on the car.
What this means: Rated load
- What it is
- The rated payload the car is designed to carry.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 1000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 500 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the rated load from the design or plate, not a typical load.
What this means: Counterweight balance (share of rated load)
- What it is
- The share of the rated load added to the car mass to set the counterweight mass.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Changing it shifts which case, full or empty, needs the most traction and the out-of-balance power.
- Get it right
- Type the percentage itself; the counterweight equals car mass plus this share of rated load.
What this means: Acceleration or braking
- What it is
- The largest acceleration or emergency braking deceleration of the car, which increases the rope tension on the heavier side.
- Units and format
- Enter a number in m/s²: metres per second squared. The form accepts 0 m/s² to 5 m/s². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 m/s² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More acceleration raises both tension ratios the sheave must grip.
- Get it right
- Use the emergency stop deceleration if it is larger than normal acceleration.
Sheave
What this means: Rope-to-groove friction coefficient
- What it is
- The basic friction coefficient between the steel rope and the sheave groove material.
- Units and format
- Enter a plain number with no unit. The form accepts 0.01 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher friction raises the traction the sheave can provide.
- Get it right
- Take it from the lift standard or sheave maker for the groove material and lubrication.
What this means: Wrap angle
- What it is
- The angle of sheave rim the ropes are in contact with.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0.000001° to 540°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 180° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More wrap raises the traction limit.
- Get it right
- A deflector sheave or double wrap changes it; measure the arc of contact.
What this means: Groove included angle (0 = round seat)
- What it is
- The included angle of a vee or undercut groove, which wedges the rope and raises the effective friction. Use 0 for a round groove seat.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 179°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A narrower groove raises effective friction and traction.
- Get it right
- Enter 0 for a plain round (U) groove; the tool then uses the basic friction coefficient.
Drive
What this means: Rated speed
- What it is
- The rated travel speed of the car, used with the out-of-balance load for motor power.
- Units and format
- Enter a number in m/s: metres per second. The form accepts 0.000001 m/s to 20 m/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 m/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Power needed rises in direct proportion.
- Get it right
- Enter metres per second.
What this means: Drive efficiency
- What it is
- The overall efficiency from motor input to the ropes, through the gearing and sheave.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 10% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 70% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower efficiency raises the motor power needed.
- Get it right
- Type the percentage itself; take it from the drive maker.
What this means: Motor power fitted
- What it is
- The rated power of the motor you intend to fit, compared with the power needed to lift the out-of-balance load.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 10000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5.5 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the motor nameplate rating for the duty cycle of the hoist.
Method and formulas
- Counterweight = car + balance × rated load.
- Tensions = mass × g on each side, with the heavier side × (g + a) ÷ g and the lighter × (g − a) ÷ g; ratio = heavier ÷ lighter, full and empty.
- Traction available e^{μ′θ} with μ′ = μ ÷ sin (groove ÷ 2) (as the belt friction tool); motor power = largest out-of-balance × g × speed ÷ efficiency.
- Example: counterweight 850 kg; ratios 1.4332 full and 1.5689 empty against 2.5056; motor 3.5024 kW.
Limitations
- Rope and compensation weights, and car friction in the guides, are not included.
- Static and acceleration traction; buffer impact and safety gear cases are separate.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check traction on a goods lift or hoist
- Choose the counterweight balance
- Check motor power for a counterweighted drive
Who it is for
- Lifting engineers
- Design engineers
- Maintenance engineers
What you get
- Counterweight and rope tensions
- Tension ratios against the traction available
- Motor power
Interpreting the result
- Balancing about half the rated load keeps the ratio about the same full and empty, which makes the best use of the traction.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 165–168, 2473–2490. Topic reference; no source tables or recommended settings are reproduced.
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