HOISTS 777 SERVICE MANUAL
5-4 Published 10-01-2012, Control # 045-08
BLOCK-UP LIMIT INSTALLATION AND
ADJUSTMENT
A block-up limit control (also called anti two-block device) is
a two-blocking prevention device which automatically stops
the load drum from hoisting and the boom from lowering
when a load is hoisted a predetermined distance.
DEFINITION: Two-blocking is the unsafe condition in
which the load block or the weight ball contacts the sheave
assembly from which either is suspended.
Two-blocking can result in failure of sheaves and wire rope,
possibly causing load to fall.
The block-up limit controls consist of the following
components (see Figure 5-3):
1. Normally closed limit switch assembly fastened at any or
all of the following locations (multiple limit switches are
wired in series):
a. Lower boom point.
b. Upper boom point.
c. Fixed jib point.
d. Luffing jib point.
e. Universal anchor joint.
2. Weight freely suspended by chain from each limit switch
actuating lever (weight encircles load line as shown in
Figure 5-4).
3. Lift block clamped to single-part load line or lift plates
fastened to multiple-part load block.
Operation
See Figure 5-3 and Figure 5-8 for component identification.
NOTE: See wiring diagrams Boom Wiring, Limits, and
Load Indicator Electrical Assembly drawings at end
of this section.
Block-Up Limit Control Deactivated
During normal operation, the weight overcomes the spring
force and rotates the actuating lever away from the limit
switch lever. This action allows the limit switch to close the
load drum up and boom/luffing jib down electric circuits. The
load can be hoisted and the boom/luffing jib can be lowered.
Block-Up Limit Control Activated
When the weight is lifted by the lift block or the lift plates,
spring force rotates the actuating lever against the limit
switch lever. This action causes the corresponding limit
switch to open the load drum up and boom/luffing jib down
electric circuits.
The load drum and boom hoist pumps stroke to off. At the
same time, the load drum and boom/luffing jib parking brakes
apply to stop the load drum from hoisting and the boom/
luffing jib from lowering.
Installation
The block-up limit control must be installed according to
Boom Wiring, Limits and Load Indicator Electrical Assembly
drawings.
Securely fasten the electric cords to the boom and jib with
the metal straps and nuts provided.
When there is more than one block-up limit switch, wire limit
switches in series. Connect the electric wires to the
normally-closed contacts inside each limit switch.
See Figure 5-4 for installation of the weights.
WARNING
Two-Blocking Hazard!
Block-up limit control is a protective device designed only
to assist operator in preventing a two-blocking condition;
any other use is neither intended nor approved.
Block-up limit control may not prevent two-blocking when
load is hoisted at maximum single line speed. Operator
must determine fastest line speed that allows block-up
limit control to function properly and not exceed that line
speed.
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