Bonfiglioli KRG/KCG/KSD Hydraulic Coupling Selection and Integration Guide

Bonfiglioli KRG/KCG/KSD Hydraulic Coupling: Professional Selection Guide for Heavy Load Starting and Overload Protection

In heavy industries such as mining, metallurgy, building materials, ports, and power, equipment such as belt conveyors, crushers, ball mills, and bucket elevators face extremely harsh start-up conditions. When starting directly, the motor needs to overcome huge static friction and load inertia, resulting in a starting current of 6-8 times the rated current, causing serious impact on the motor, reducer, and power grid. Bonfiglioli's KRG, KCG, and KSD series hydraulic couplings (fluid couplings) are the core transmission components designed to solve this problem. This article will start from engineering practice and provide a detailed analysis of the working principle, selection process, thermal verification calculation, and technical differences among various series of hydraulic couplings.

Working principle and core advantages of hydraulic coupling

Hydraulic coupling belongs to hydraulic transmission components, and its core structure includes:

Pump impeller (drive impeller): rigidly connected to the input shaft (motor side)

Turbine (driven impeller): rigidly connected to the output shaft (load side)

Shell: Connected to the external impeller flange to form a sealed working chamber

When the motor drives the pump wheel to rotate, the oil in the working chamber flows from the inside to the outside of the pump wheel under the action of centrifugal force, impacts the turbine blades, transfers kinetic energy to the turbine, and drives the output shaft to rotate. The oil then flows back radially inward to the pump impeller, forming a circulation.

Key feature: The hydraulic coupling has no mechanical rigid connection and relies entirely on oil to transmit power.

1. Wear free transmission

Due to the absence of mechanical contact between the pump impeller and the turbine, theoretically there is no mechanical wear, and the service life is extremely long, requiring only regular replacement of oil and seals.

2. Smooth start-up and current peak shaving

The motor is almost in an unloaded state when starting - the hydraulic coupling transmits very little torque at low speeds, allowing the motor to quickly accelerate to near the rated speed. Only when the motor approaches the rated speed, the coupling gradually establishes torque and smoothly accelerates the load. Compared with direct start-up, the peak current can be reduced from 6 times the rated current to below 2.5 times, significantly reducing the impact on the power grid and electrical equipment.

3. Overload protection and isolation

When the load becomes stuck or overloaded, the slip between the pump impeller and the turbine increases sharply, and the internal oil temperature of the coupling rises rapidly. When the temperature reaches the safe threshold (140 ° C or 190 ° C), the fusible plug melts, the oil is discharged, and the motor automatically disengages from the load, effectively preventing motor burnout and equipment damage.

4. Load balancing driven by multiple motors

In situations where dual motors drive the same load (such as long-distance belt conveyors), the hydraulic coupling can automatically balance the output torque of the two motors without the need for complex electrical synchronization control.

Delay Chamber Technology: The Secret to Extreme Smooth Startup

For high inertia loads such as large belt conveyors and ball mills, even the standard hydraulic coupling starting torque (about 200% of the rated torque) may still be too high. Bonfiglioli offers a delay chamber option that further reduces the starting torque to 150% (single delay chamber) or even 120% (dual delay chamber).

Working Principle:

In a stationary state, a portion of the total oil volume is stored in the delay chamber, reducing the effective oil volume in the working chamber and thus lowering the starting torque.

As the speed increases, under the action of centrifugal force, the oil in the delay chamber gradually flows into the working chamber through the calibration nozzle.

When the coupling reaches the rated speed, all the oil enters the working chamber and transmits the rated torque with the minimum slip (1.5%~6%).

Selection guidance:

Single delay cavity (code C): suitable for medium inertia loads, optional starting from Size 11.

Double delay cavity (code CC): Suitable for situations with extremely high inertial loads or extreme smooth start-up requirements, available from Size 15 onwards.

Technical Differences and Selection of KRG/KCG/KSD Series

1. KRG series (standard type)

The most universal hydraulic coupling series, with an integrated elastic coupling on the output shaft, can be directly connected to the input shaft of the gearbox. Provide rich options:

The input shaft can be customized according to the motor shaft diameter or provide guide holes (FP option), with on-site drilling

Optional disc brake (D_) or drum brake (F_)

Configurable single delay cavity (C) or dual delay cavity (CC)

Installation direction: Horizontal (OR), Vertical Upward (VA), Vertical Downward (VB)

2. KCG series (quick release type)

KCG integrates gear couplings at both the input and output ends based on KRG. Its core advantages lie in:

During maintenance, the hydraulic coupling body can be extracted without moving the motor or reducer

Significantly reduce maintenance downtime

No need to realign, maintain the original axis accuracy

Suitable for occasions that require frequent maintenance or replacement of couplings

3. KSD series (pulley type)

KSD is designed for installation scenarios where the motor and gearbox shafts are not aligned (parallel axes). Its output end is a reserved interface for installing the pulley (the pulley is not randomly provided), and users can choose a standard triangular pulley according to the speed ratio. KSD also supports single/double delay chambers, but it should be noted that delay chambers cannot be selected for vertical installation.

Selection process and technical verification

1. Preliminary selection (based on power and speed)

According to the selection diagram in the user manual (see Fig. 7 in the original text), the base size (Size 8-24) of the hydraulic coupling can be preliminarily determined based on the motor power (kW) and input speed (min ⁻¹). If the selection point falls on the boundary between two curves, it is recommended to choose a larger machine base size and reduce oil filling to achieve lower temperature rise and longer service life.

2. Start time calculation (t_acc)

The startup time is calculated according to the following formula:

tacc=two π⋅nMOT⋅JLOAD sixty⋅MACC[s]

t acc= 60⋅M ACC 2π⋅n MOT⋅J LOAD [s]

among which

JLOAD

Convert the load to the total inertia of the motor shaft (including couplings and reducers)

MACC

Acceleration torque=average coupling torque - load torque (to be found through the slip torque curve of the coupling)

Engineering experience value: The startup time is generally controlled within 5-20 seconds. Too long may cause the oil temperature to exceed the standard, while too short may lose the meaning of smooth start-up.

3. Temperature rise verification (Δ T)

Hydraulic couplings generate heat during start-up and operation, and it is necessary to verify whether the temperature rise is within a safe range.

Temperature rise during startup phase (Δ T-acc):

ΔTacc=QaccTC[°C]

among which

Qacc

The heat generated during the acceleration phase (which can be obtained by integrating the torque slip curve),

TC is the total heat capacity of the coupling.

Steady state operating temperature rise (Δ T_uts):

ΔTuts=Ploss⋅tutsTC[°C]

among which

Ploss

For steady-state slip loss (approximately 2% to 6% of rated power).

End temperature of cycle:

Tuts=Tamb+ΔTacc+ΔTuts[°C]

This value must be lower than the maximum allowable operating temperature of the coupling (usually 120 ° C~140 ° C).

4. Maximum allowable startup times (Z-MAX)

If the application needs to be started frequently (such as devices that work periodically), the maximum allowed number of starts per hour needs to be calculated:

ZMAX=TC⋅(TMAX−Tamb)−Ploss⋅tutsQacc

Z MAX= Q acc TC⋅(T MAX−T amb)−P loss⋅t uts

If the actual number of starts exceeds Z-MAX, the coupling will accumulate overheating and eventually trigger the melting plug protection. The solution includes choosing a larger machine base size, adding delay chambers, extending the start-up interval, or adding external cooling.

Security options and maintenance points

1. Fusible plug (standard configuration)

All KRG/KCG/KSD couplings come standard with a melting plug set at a temperature of 140 ° C. When the oil temperature exceeds the standard, the melting plug melts, the oil is discharged, and the motor is disconnected from the load. During installation, a protective barrier must be installed below the coupling to prevent high-temperature oil from splashing and injuring people.

2. Switch sales+relay (SP option)

For situations where oil leakage is not allowed (such as food processing, clean environments), the Switching Pin option can be selected:

Internally equipped with easy to fuse gold rings, the spring-loaded pin is released at 120 ° C

Pin extends 16mm, triggering external micro switch (provided with coupling)

Relay contacts can be connected to alarm circuits or motor control circuits to achieve electrical protection without oil leakage

After troubleshooting, simply replace the melting ring to reset, without the need to replace the entire melting plug

3. Maintenance points

First oil filling: The coupling does not come with oil when it leaves the factory. Users need to add the specified brand and quantity of oil according to the installation position and operating conditions in the manual.

Sealing element: standard Viton ®  Fluororubber seal, resistant to high temperature and aging.

Anti corrosion treatment: Both cast iron and steel components are treated with anti-corrosion coatings, suitable for humid or corrosive environments.

Installation direction: All options and oil quantities are based on the specified installation position (horizontal/vertical). Changing the installation direction may require recalculating the oil quantity and may affect the functionality of the delay chamber.

Selection Example

Operating parameters:

Motor power: 55 kW, speed 1475 min ⁻¹

Speed ratio of reducer: 48.8, efficiency 0.91

Load inertia: 82000 kgm ² (converted to output shaft)

Environmental temperature: 30 ° C

Start frequency: 3 times per hour

Selection steps:

Based on P=55kW and n=1475 min ⁻¹, the preliminary selection chart shows that it is Size 15.

Considering the high inertia load, it is recommended to choose a dual delay chamber (CC) to limit the starting torque to below 150%.

Calculate the load inertia converted to the motor shaft

JLOAD=JRED+JLOAD_ref

And calculate the startup time based on this tacc

Calculate the startup temperature rise Δ Tacc and steady-state temperature rise Δ Tuts, and confirm that Tuts is less than the safety limit of 140 ° C.

Calculate Z-MAX and confirm that the actual startup frequency of 3 times per hour is within the allowable range.

If Z-MAX is insufficient, it needs to be increased to Size 17 or external cooling measures need to be added.

Final selection: KRG 15-CC-D_ (with disc brake), input shaft with guide hole (FP), output shaft with hole according to reducer shaft diameter, horizontal installation (OR), optional SP switching pin protection.