How to Replace Old Geared Motors: 12 Critical Parameters to Check

How to Replace an Old Geared Motor: 12 Parameters to Check

Replacing an old geared motor is not as simple as finding a device with the same power and gear ratio. Two geared motor units with the same kW rating may still differ in output speed, permissible torque, shaft dimensions, center height, mounting type, shaft orientation, and electrical specifications.

If information is missing during inspection, the new device may not fit the machine, run at the wrong speed, lack starting torque, or cause misalignment of gears, pulleys, and couplings. Before placing an order, it is necessary to clearly define the scope of replacement and fully verify the 12 groups of parameters below.

Should you replace the motor, the gearbox, or the entire unit?

A geared motor consists of two main components:

• Electric motor to generate rotational motion.

• Gearbox to reduce speed and transmit torque to the output shaft.

Depending on the condition of the equipment, one of three options can be chosen:

Do not assume that a motor with the same power can be mounted directly onto an old gearbox. Flanges, input shafts, drive gears, or adapter sets may vary between designs.

1. Model and nameplate information

The first step is to record all legible information on the motor and gearbox nameplates:

• Model

• Power

• Voltage

• Frequency

• Current

• Motor speed

• Gear ratio

• Output speed

• Mounting type

• Brake voltage (if applicable)

• Protection class

• Duty cycle
  

  

DSK geared motor nameplate model MFG24D-10RS0.75-4, power 0.75 kW, 4 poles, gear ratio 1/10, voltage 220/380 V, and frame 80.

It is recommended to take separate photos of the motor nameplate, the gearbox label, and an overall view of the drive assembly. The overall view helps in identifying the motor position, shaft orientation, mounting type (foot or flange), and available installation space.

The old model is important data but not the sole basis for selection. If the load, capacity, or operating time has changed, it is necessary to recalculate rather than simply copying the previous configuration.

2. Application and load characteristics

It is necessary to identify what equipment the geared motor is being used for:

• Conveyor

• Screw conveyor

• Agitator

• Feeder

• Winch drum

• Packaging machine

• Lifting mechanism

• Reversible system

Then, evaluate the load characteristics:

• Constant or variable load

• Presence of shock loads

• No-load or full-load start

• Risk of mechanical jamming

• Frequent reversing

• Operating hours per day

• Number of starts per hour

If the old equipment failed due to overload, replacing it with the exact same model may lead to recurring failures. It is necessary to check if the machine load has increased, if the material has changed, or if the operating cycle is more demanding than the original design.

3. Motor power

Motor power is usually expressed in kW or HP. When replacing, it is necessary to check:

• Rated power

• Full-load current

• Starting current

• Overload condition

• Actual mechanical power required by the machine

Do not automatically increase the power just because the old motor was running weakly. A larger motor may transmit torque exceeding the load capacity of the gearbox, shaft, gears, chains, or couplings.

Before increasing the power, check for other causes:

• Low supply voltage

• Brake not fully released

• VFD current limit

• Acceleration time too short

• Jammed load

• Seized bearings or gearbox

• Inappropriate gear ratio

Correct power does not necessarily mean the drive system is correctly selected.

4. Motor speed, number of poles, and frequency

Two motors with the same power but different numbers of poles will have different rated speeds. This directly changes the output speed and torque of the geared motor assembly.

It is necessary to cross-reference:

• Rated speed on the nameplate

• Number of motor poles

• 50 Hz or 60 Hz frequency

• Direct-on-line motor or VFD-driven

• Actual operating frequency

Do not use synchronous speed as a substitute for rated speed. Asynchronous motors have slip, so the actual speed is usually lower than the synchronous speed and varies with the load.

If the old motor was driven by a VFD, it is necessary to determine the actual operating speed of the machine instead of just reading the rated speed on the label.

5. Gear ratio and output speed

The fundamental relationship between motor speed and output speed is:

n₂ = n₁ / i

Where:

• n₁: motor speed, rpm

• n₂: output speed, rpm

• i: gear ratio

For example, a motor with a speed of 1,400 rpm combined with a gear ratio of 35:

n₂ = 1,400 / 35 = 40 rpm

This is only the theoretical speed. The actual speed depends on the motor's rated speed and load conditions.

When replacing an old gear motor, prioritize the output speed that the machine actually requires. Even with the same gear ratio, if the new motor has a different speed, the output shaft speed will also change.

If the gear ratio cannot be read, you can:

• Measure the output speed while the machine is still operating

• Check machine documentation or drawings

• Determine it from the production line capacity

• Rotate the input shaft and count the output revolutions after the device has been safely disassembled

6. Output Torque and Service Factor

For the same power, the lower the output speed, the higher the torque.

When the mechanical power at the specific shaft in question is known, torque can be estimated using the formula:

T = 9,550 × P / n

Where:

• T: torque, Nm

• P: mechanical power, kW

• n: speed, rpm

If using the motor power to estimate torque at the gearbox output, transmission efficiency must be taken into account.

In addition to operating torque, check for:

• Starting torque

• Peak torque

• Shock loads

• Number of starts

• Number of reversals

• Operating time

• Required Service Factor

A gearbox may have sufficient torque during steady-state operation but can still be overloaded during starting, reversing, or load jamming.

Do not apply a fixed Service Factor value to all applications. Selection must be based on load characteristics, duty cycle, and the load-bearing capacity of the specific model.

7. Gearbox Type and Output Shaft Orientation

It is necessary to determine the configuration of the old gearbox:

• Coaxial

• Parallel shaft

• Right-angle drive

• Solid shaft

• Hollow shaft

• Single output shaft

• Double output shaft

For right-angle or parallel shaft gearboxes, the output shaft orientation is particularly important. A new device may have the correct power and gear ratio but still cannot be installed if the shaft is on the wrong side or the motor faces the machine frame.

When submitting a replacement request, it is recommended to photograph the device from:

• The motor side

• The output shaft side

• The top

• Both sides of the device

• The entire mounting position on the machine

8. Mounting Style and Installation Position

Common configurations include:

• Foot mounting

• Flange mounting

• Shaft mounting

• Horizontal mounting

• Vertical mounting

• Motor positioned above, below, or to the side of the gearbox

The mounting position not only relates to mechanical space but also affects:

• Oil level

• Breather plug position

• Oil drain position

• Lubrication capability

• Oil seals

• Bearings

Do not attempt to convert a gear motor from horizontal to vertical mounting without verifying the lubrication configuration of the new model.

For replacement equipment, the operating position must be clearly defined at the time of model selection and ordering.

9. Output shaft dimensions and keyway

Shaft dimensions determine the compatibility with existing sprockets, pulleys, couplings, or machine shafts.

For solid shafts

Required measurements:

• Shaft diameter

• Length of the protruding shaft section

• Keyway dimensions

• Key length

• Distance from the shaft shoulder to the shaft end

• Threads or center holes if applicable

For hollow shafts

Required checks:

• Bore diameter

• Mounting length

• Keyway dimensions

• Machine shaft diameter

• Torque arm

• Clearance for installation and removal

Two shafts with the same diameter but different lengths or keyways may still be incompatible with existing transmission components.

Avoid force-fitting or arbitrary shaft machining without verifying structural integrity and torque transmission capability.

10. Foot, flange dimensions, and center height

These parameters determine whether the new gear motor will fit the existing mounting location.

For foot-mounted types

Required measurements:

• Longitudinal bolt hole spacing

• Transverse bolt hole spacing

• Hole diameter

• Foot length and width

• Height from the base to the shaft center

• Distance from the gearbox center to the shaft end

For flange-mounted types

Required measurements:

• Flange outer diameter

• Pilot diameter

• Bolt circle diameter

• Number of bolt holes

• Hole diameter

• Shaft length

• Distance from the flange face to the shaft end

Similar external appearance does not imply equivalent dimensions. Always cross-reference drawings before finalizing replacement equipment.

If machine base modification is required, ensure proper center height, alignment, and structural rigidity.

11. Voltage, frequency, and connection type

Simultaneous verification is required for:

• Rated voltage

• Frequency

• Number of phases

• Rated current

• Star or delta connection type

• Rated speed

• Actual power supply at the facility

Do not consider voltage independently of frequency. Motors with the same voltage but designed for different frequencies may result in different speeds and operating conditions.

If using a VFD, additional checks are required:

• Input and output voltage

• Motor rated current

• Base frequency

• Operating frequency range

• Control mode

• Acceleration and deceleration time

After replacing the motor, the parameters in the VFD must also be updated according to the new equipment's nameplate.

12. Brakes, encoders, and operating conditions

If the old geared motor has accessories, it is necessary to accurately identify:

• Electromagnetic brake

• Brake coil voltage

• Brake rectifier

• Encoder

• Forced cooling fan

• Backstop

• Thermal switch

• Speed sensor

Additionally, it is necessary to check the operating conditions:

• Daily operating hours

• Number of starts per hour

• Reversible operation

• Ambient temperature

• Dust, moisture, or water

• Indoor or outdoor installation

• Direct-on-line or VFD-driven operation

• Requirement for load holding during power failure

If the old motor has a brake, the correct brake voltage and switching method must be determined. If the motor operates at low speeds for extended periods, torque and cooling capacity must be verified.

Procedure for replacing old geared motors

Step 1: Capture complete equipment information

It is necessary to clearly photograph:

• Motor nameplate

• Gearbox label

• General view

• Output shaft

• Foot mount or flange

• Sprocket, pulley, or coupling

• Mounting position on the machine

The general view photo should show the motor orientation, shaft direction, and installation clearance. Sending only the nameplate photo is often insufficient to verify replacement compatibility.

Step 2: Determine the reason for replacement

Geared motors may be replaced due to:

• Motor burnout

• Gear damage

• Bearing wear

• Oil leakage

• Insufficient torque

• Obsolete spare parts

• Productivity increase

• Machine retrofitting

If the cause is overload, misalignment, mechanical jamming, or weak machine structure, it must be addressed before installing the new equipment.

Step 3: Measure actual dimensions

Do not estimate dimensions from images. It is necessary to measure directly:

• Shaft diameter and length

• Keyway dimensions

• Foot hole spacing

• Center height

• Flange dimensions

• Pilot diameter

• Installation space

• Clearance for disassembly and maintenance

If machine drawings or old equipment drawings are available, they should be used in conjunction with actual measurements.

Step 4: Verify parameters before selecting a new model

Before finalizing the replacement geared motor, it is necessary to confirm:

• Power rating

• Voltage and frequency

• Motor speed

• Gear ratio

• Output speed

• Operating torque

• Starting torque

• Gearbox type

• Output shaft orientation

• Mounting type

• Shaft dimensions

• Brake and accessories

The new equipment does not necessarily have to be identical in appearance, but parameters affecting operation and installation must be compatible.

Step 5: Post-installation inspection

After installing the new gear motor, it is necessary to check:

• Direction of rotation

• Concentricity

• No-load and load current

• Output speed

• Vibration

• Noise

• Temperature

• Brake operation

• Oil level

• Leakage status

If a VFD is used, it is necessary to enter the correct motor parameters and set appropriate acceleration and deceleration times before running under load.

Frequently Asked Questions

Can I replace a gear motor with a different model but the same power rating?

It may be considered, but you must also compare the speed, torque, gear ratio, shaft orientation, mounting type, dimensions, and electrical parameters.

If the gear ratios are the same, will the output speeds be the same?

Not necessarily. The output speed also depends on the rated speed of the motor and the operating frequency.

Should I choose a new motor larger than the old one?

Only increase the power after recalculating the load and checking the torque capacity of the gearbox and the entire transmission mechanism.

What should I do if the nameplate is unreadable?

You need to measure the dimensions, identify the application, load, output speed, and power supply. You can refer to machine drawings or measure the speed while the system is still operational.

If the new motor does not fit the base, can the machine base be modified?

It is possible in some cases, but you must check the center height, concentricity, maintenance space, and the rigidity of the base.

Is it necessary to replace the gears or couplings?

They must be replaced or re-machined if the shaft diameter, shaft length, or keyway of the new equipment is incompatible.

MDriveTech supports old gear motor replacement

MDriveTech provides support for inspecting and selecting gear motors, DSK, Nara Samyang, and NK gearboxes for replacement or machine retrofitting needs.

Customers should send photos of the nameplate, installation position, shaft dimensions, output speed, application, and load conditions for configuration matching before placing an order.

Hotline: 0868 789 647
Email: [email protected]

Conclusion

When replacing an old gear motor, do not choose solely based on power or gear ratio. The replacement equipment must simultaneously meet the requirements for speed, torque, Service Factor, gearbox type, mounting position, shaft orientation, mechanical dimensions, and electrical parameters.

Measuring actual dimensions and checking drawings before ordering helps prevent issues such as the new equipment not fitting, running at the wrong speed, or lacking torque. If the old gear motor failed due to overload or improper installation, the root cause must be addressed before putting the new unit into operation.