Reference
Only
MAINTENANCE
2250 MAX-ER 2000 OPERATOR’S MANUAL
6-10
Published 01-08-13, Control # 049-05
BLOCK-UP LIMIT CONTROL
General
The 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 blo ck - up limit c ont rol c ons is ts of t he following
components (see Figure 6-10):
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.
2.
Weight freely suspended by chain from each limit switch
actuating lever (weight encircles load line).
3.
Lift block clamped to single-part load line or lift plates
fastened to multiple-part load block.
Operation
See Figure 6-10 and Figure 6-13 for following procedures.
See Section 3 of your Crane Service Manual for an electric
schematic of the block-up limit control system.
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 DOWN electric circuits. Therefore,
the load can be hoisted and the boom 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 DOWN electric
circuits.
The load drum and boom hoist pumps stroke to off. At the
same time, the load drum and boom parking brakes apply to
stop the load drum from hoisting and the boom from
lowering.
The BLOCK UP fault alarm (in operator’s cab) will come on
to alert the operator.
Installation
The system must be installed according to the Block-Up Limit
Control Assembly Drawing at the end of Section 4 in this
manual. Make sure weights are suspended from proper
lengths of chain or system will not operate properly.
Securely fasten the electric cords to the boom and jib with
the nylon straps and cord clips provided.
Connect the electric wires to the normal-closed contacts
inside each limit switch.
See Figure 6-12 for installation of the weights.
Storing Electric Cord
The electric cords for the boom and fixed jib are long enough
to accommodate the maximum length of each attachment.
Store the excess cord for the boom on the reel mounted on
the boom butt (Figure 6-11).
The reel is equipped with a locking pin. Disengage the
locking pin to allow the reel to be wound. Engage the locking
pin to lock the reel in position. The power supply cord to the
reel must be disconnected before the reel can be wound.
Store the excess electric cord for the fixed jib by winding the
cable around the brackets on the jib butt.
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 will allow block-up
limit control to function properly and, thereafter, not
exceed that line speed.
