Reference
Only
HOISTS
MLC650 SERVICE/MAINTENANCE MANUAL
5-62
Published 09-09-16, Control # 229-09
BLOCK-UP LIMIT SWITCH
A block-up limit (also called anti-two-block device) is a two-
blocking prevention device that stops the load drum from
hoisting and the boom from lowering when a block or hook is
too close to the sheave.
Details of the block-up limit system are found on the boom
ESI assembly drawings.
NOTE:
The block-up limit is a protective device designed
only to assist the operator in preventing a two-
blocking condition. Any other use is neither
intended nor approved.
for the following information.
The block-up limit system consists of the following
components:
•
A limit switch, wired for normally closed operation,
fastened at the following locations:
-
Lower boom point
-
Upper boom point
•
A weight freely suspended by a chain from each limit
switch actuating lever (weight encircles load line as
shown)
•
A lift block fastened to the load line, or a lift plate
fastened to the load block
For service of the block-up limit components in the luffing jib,
see the MLC650 Luffing Jib Operator Manual supplied with
the luffing jib.
Switch Activation Overview
For a complete wiring diagram of the system, see the ESI
Boom Wiring Assembly Drawing for each block-up limit
assembly.
Normal conditions (block-up limit control deactivated)—
During normal operation, the weight overcomes spring force
and rotates the actuating lever away from the limit switch
lever. This allows the limit switch to close the hoist control
handle electrical circuits.
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 causes the
corresponding limit switch to open the hoist control handle
electrical circuits.
The load drum and the boom/mast hoist pumps will stroke to
off. At the same time, the load drum and the boom park
brakes apply to stop the load drum from hoisting and the
boom from lowering.
Weekly Maintenance
Inspect and test the block-up limits weekly or every 40 hours
of operation, as follows.
1.
Lower the boom (and the jib, if equipped) onto blocking
at the ground level.
2.
Lockout-tagout the crane.
3.
Carefully inspect the following items.
a.
Inspect each weight for freedom of movement on
the load line.
b.
Inspect each weight, chain, shackle and connecting
pin for excessive or abnormal wear. Make sure the
cotter pins for the shackles are installed and spread.
c.
Inspect the entire length of each electrical cable for
damage.
d.
Make sure the electrical cables are clear of all
moving parts on the boom (and the jib, if equipped).
e.
Make sure the cables are securely fastened to the
boom (and the jib, if equipped) with nylon straps.
f.
Make sure all electrical cable connections are
securely fastened.
WARNING
Two-Blocking Hazard!
Two-blocking is the condition in which the load block or
the weight ball runs into the boom sheaves.
Two-blocking can result in failure of the sheaves and wire
rope, possibly causing the load to fall.
The operator shall determine the fastest line speed that
allows the block-up limit to function properly and
thereafter not exceed that line speed.
If the block or weight ball approaches the boom sheaves
too fast, the block-up limit may not prevent two-blocking.
CAUTION
Avoid Machinery Damage!
If inspection reveals a problem with the block-up limit
components, do not operate the crane until the block-up
limit has been repaired to proper working order.
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