ABB Mechanism HMB-8, Instructions Manual

4 November 2005

641P026-008
HMB-8 / HMB-8.7 Mechanism

© 

(formerly HMB-11)

Page 6

HMB-8  Mechanism

1.

Introduction

This module provides a description of components and oper-
ating principles as well as instructions for commissioning,
operating, and maintaining the HMB-8 mechanism.

1.1

Description

The HMB mechanism is a compact, hydraulically-operated
device which uses a compressible stack of disc springs as
an energy storage system or accumulator.  The mecha-
nism receives operating signals from the electrical control
system and translates these signals via open and close
pilot valves into linear mechanical motion which respec-
tively open and close the interrupter contacts.

A hydraulic pump draws oil from a low pressure volume and
compresses the oil into a high pressure volume which
drives three accumulator pistons to compress the disc
spring assembly.  During breaker operation, stored energy
is released from the disc spring assembly 51008 (Fig.1) to
the hydraulic system via a changeover valve responding to
a signal from the open or close pilot valve.  This energy is
transferred to the drive piston of the mechanism causing it
to move into either the OPEN or CLOSE position.  There is
no direct mechanical link between the disc spring assembly
and drive piston.  When the breaker is operating, the disc
spring assembly discharges in increments, releasing only
the amount of energy required for an OPEN or CLOSE
operation.  The total energy storage capacity (without re-
charging) of a fully compressed disc spring assembly
allows for two open operations, (O-CO)  for HMB-8.3 or HMB-
8.7 mechanisms; and (CO-CO) for HMB-8.2 mechanisms.
The hydraulic pump is controlled by a limit switch assembly
97612 (Fig. 1) which monitors the degree of compression
of the disc spring assembly and automatically re-charges
the disc spring assembly as energy is consumed.  By
monitoring the degree of compression of the disc spring
assembly, the limit switch assembly also provides low
spring charge energy alarms and lock-outs should the
system energy drop critically low due to failure to re-charge.
The HMB design monitors energy storage 

only by spring

compression (spring travel) 

and does not include hydraulic

pressure gauges and pressure switches. A mechanical
pressure relief valve will release the oil pressure if the limit
switches fail to turn off the pump motor.

The velocity of the mechanism for both OPEN and CLOSE
operations is factory-set (for the breaker application).  An
externally  visible position indicator 51033 (Figures 1 and 1b)
detects the breaker position. The position indicator is oper-
ated by a lever system directly coupled to the drive piston.  The
mechanism covers are made of painted fiberglass material
and are divided into two removable halves.

Onboard auxiliary switches 97604 (Figures 1 and 1b) are
included that are pre-adjusted and fully wired to waterproof
wiring connectors 97606 (Fig.1b) on the mechanism body.

The integrated design affords a compact unit that is essen-
tially maintenance-free.  By virtue of its integrated design,
the hydraulic spring drive has a minimal amount of high
pressure hydraulic connections and no piping connections.

The low pressure hydraulic circuit is sealed from atmo-
sphere by very reliable static and dynamic seals.

A split, bolted coupling is used to attach the drive (or output)
shaft of the mechanism to the operating rod of the circuit
breaker linkage or pole unit.

2

Hydraulic Elements

The hydraulic components on the HMB mechanism in-
clude:

• Pump motor 51002 (Figs. 1 & 5) drives the pump

element 51014 (Fig. 5);

• Pump element 51014 (Fig. 5) pumps hydraulic oil

from the low pressure volume into the high pressure
volume, compressing the disc spring assembly
51008 (Fig. 1);

• Pump limit switch and cam assembly 51046 (Fig. 4)

operates the limit switches 97612 (Fig. 1) and deter-
mines how far the disc spring assembly travels.  The
energy storage level for the HMB mechanism is
solely determined by the amount of distance the disc
spring assembly is compressed.  There are no
hydraulic pressure gauges or pressure switches.
Instead,  limit switches start and stop the spring
charging motor and provide alarms for insufficient
energy levels based on the amount of  compression
of the disc spring assembly.  The pump limit switch
and cam assembly controls the following:
- Pump motor (on/off)
- Low spring charge alarm
- Low spring charge lock-out (trip block)
- Close-Open operation block

• Pressure equalizing valve 51017A (Fig. 2) connects

the high pressure section with the low pressure sec-
tion and acts as a gate to permit deliberate reduction
of hydraulic pressure in the mechanism; it also actu-
ates if the limit switch fails during pump operation;

• Oil drain valve 51013 (Fig. 1), when opened, is used

to drain or fill hydraulic oil in the mechanism;

• Check valve 51059 (Fig. 5) prevents oil from

backflowing from the high pressure section to the low
pressure section when the pump element is at rest;

• Filter 51079 (Fig. 5) removes particles from hydraulic oil;
• Pilot valves (close) 51004 (Figs. 2 & 8) and (open)

51005 convert electrical commands to hydraulic
commands; after actuation, the pilot valve returns to
its initial position;

• Changeover valve 51006 (Fig. 2 & 8) allows hydraulic

oil to move the drive piston 51012 (Fig. 7) by changing
pressure on the back side of the drive piston;

• The low pressure section of the HMB hydraulic

spring mechanism as shown in Figure 2 serves as
an oil reservoir.

• Heater 51150 (Fig. 1) within the mechanism pre-

vents condensation in the mechanism;

• Sight windows are located in the mechanism cover

51028 (Fig. 1); the oil level indicator 51036 (Fig.1) can be
seen through one window; the spring charge indicator
97611 (Fig.1b) can be seen through the other window.