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: