What Causes A Chain Pulley Block To Slip Under Load?

By Author: Indolift
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A chain pulley block is designed to lift and hold a suspended load without uncontrolled movement. When a load begins to slip, creep downward, or move after the operator stops pulling the hand chain, it is a warning that something in the hoist, brake system, load chain, or lifting arrangement needs attention.
Load slippage should never be treated as a minor operating issue. A slipping load can become unstable and, depending on the circumstances, may result in uncontrolled lowering or a dropped load.
The most common causes include brake contamination or wear, damaged brake components, overloading, incorrect maintenance, worn load-chain or load-wheel components, and improper use of the hoist. Inspection and repair should follow the manufacturer's instructions and applicable lifting-equipment requirements.
What Does “Slipping Under Load” Mean?
In a manual chain pulley block, the braking mechanism is intended to hold the load when the operator stops operating the hand chain. If the suspended load gradually moves downward without the operator intentionally lowering it, the hoist may be experiencing load ...
... slippage.
This is different from normal controlled lowering.
For example, if an operator stops pulling the hand chain and a suspended load continues to descend, even slowly, the brake is not holding the load as intended.
A properly functioning hoist should not allow uncontrolled movement of a suspended load.
The braking system is therefore one of the most important components of a manual chain pulley block. OSHA's crane requirements similarly emphasize holding brakes, inspection of brake components, and testing brakes when handling loads approaching rated capacity. (OSHA)

1. Worn or Damaged Brake Components
One of the first things to investigate when a chain pulley block slips is the braking mechanism.
Manual chain hoists commonly use friction-based braking arrangements. Depending on the design, the brake assembly may contain components such as friction discs, pressure plates, springs, ratchets and pawls.
With repeated use, braking surfaces and mechanical components can wear.
As wear progresses, the brake may no longer generate sufficient holding force to keep the load stationary.
Signs may include:
• The load gradually creeps downward.
• The hoist does not hold the load immediately after operation stops.
• Braking performance becomes inconsistent.
• Unusual sounds occur during lifting or lowering.
• The problem becomes more noticeable with heavier loads.
Brake components should not simply be adjusted or replaced based on appearance. The hoist manufacturer's inspection and repair procedure should be followed.
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2. Oil or Grease Contamination on the Brake
Another important cause is contamination of the braking surfaces.
This can happen when lubrication is applied incorrectly during maintenance. Lubricant that is appropriate for the load chain or other moving components should not automatically be applied to the brake.
Some manual chain hoist designs use dry friction braking surfaces. Manufacturer manuals specifically warn that oil or grease on these surfaces can cause the brake to malfunction or slip
This is why maintenance should distinguish between:
Components that require lubrication
and
Brake surfaces that must remain clean and dry.
Simply adding more lubricant when a chain pulley block develops a problem can make the situation worse.

3. Rust, Corrosion or Dirt in the Brake Mechanism
Lifting equipment used in humid, dusty, corrosive or outdoor environments can develop corrosion or contamination inside mechanical components.
Rust can affect the movement and engagement of brake components. Dirt and foreign material can also interfere with proper contact between braking surfaces.
A manufacturer troubleshooting guide identifies rust, dirty braking surfaces and oily braking surfaces among potential causes of load slipping
The risk can increase when a chain pulley block is:
• Stored outdoors
• Exposed to moisture
• Used in coastal environments
• Used around chemicals
• Left unused for extended periods
• Poorly maintained
A hoist that has developed corrosion in critical braking components should be inspected rather than returned to service simply because the exterior looks acceptable.
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4. Overloading the Chain Pulley Block
A chain pulley block should never be used beyond its rated capacity or Working Load Limit (WLL).
Overloading places excessive forces on the hoist's load-bearing and braking components. It can contribute to premature wear or damage and may compromise the equipment's ability to control the load.
Overloading can also occur without the operator intentionally exceeding the rated capacity. For example, a load may be heavier than estimated, or the load may become stuck and require considerably greater force to start moving.
OSHA states that cranes should not be loaded beyond their rated load and also requires loads to be properly balanced before being lifted. (OSHA)
Never use chain-pulling effort as an indication that a load is within capacity.
The actual load weight should be established before lifting.
5. Worn or Damaged Load Chain
The load chain is another component that deserves close inspection.
A load chain subjected to heavy use can develop:
• Wear
• Elongation
• Bent or twisted links
• Gouges or nicks
• Corrosion
• Cracks
• Other deformation
A damaged chain can also affect how the chain engages with the load sheave.
Inspection guidance for hoists calls for checking load chains for conditions such as excessive wear, twisting, distorted links and excessive stretch. (OSHA)
If the chain does not run correctly through the load sheave, lifting performance can become abnormal.
A damaged load chain should not be repaired by improvising individual links or substituting an incorrect chain. Replacement should be carried out according to the manufacturer's specification.
6. Worn Load Sheave or Incorrect Chain Engagement
The load chain has to engage correctly with the load sheave.
If the chain and load sheave are excessively worn, contaminated or mismatched, the chain may not seat correctly.
This can produce symptoms such as:
• Chain jumping
• Cracking or unusual noises
• Jerky lifting
• Difficult operation
• Irregular chain movement
Inspection guidance notes that chain jumping can indicate problems involving the chain, load wheel or debris in the load-wheel pockets.
The correct chain specification is particularly important. Installing an incorrect load chain can create an improper engagement between the chain and load sheave.

7. Incorrect Chain Installation or Twisting
On multi-fall chain pulley blocks, incorrect chain reeving can create serious operational problems.
The load chain needs to run through the hoist correctly and without twisting. A twisted or improperly installed chain can interfere with smooth operation and correct engagement.
Before using a chain pulley block, the chain should be checked for:
• Twisted links
• Kinks
• Incorrect reeving
• Damage
• Abnormal positioning around the load sheave
Pre-use inspection guidance also emphasizes checking correct reeving on multi-fall chain blocks.

8. Excessive Shock Loading
A chain pulley block is intended to lift loads in a controlled manner. Shock loading occurs when the equipment is subjected to a sudden increase in load force.
For example, suddenly pulling on the hand chain to free a stuck load can generate forces considerably different from those experienced during a normal controlled lift.
Repeated shock loading can contribute to damage or accelerated wear of mechanical components.
For lifting operations, the load should be raised smoothly rather than jerked from the floor.
OSHA guidance similarly calls for avoiding sudden acceleration or deceleration during load movement and requires the load to be properly secured and balanced before lifting.

9. Improper Lifting Arrangement
Sometimes what appears to be a chain pulley block problem is actually caused by the lifting arrangement.
The load should be properly balanced before it is raised. If the lifting point is significantly offset from the load's centre of gravity, the load can tilt or rotate.
An unstable load can introduce unusual forces into the lifting arrangement and make the operation harder to control.
The hook should be positioned appropriately over the intended lifting point, and the lifting arrangement should be assessed before raising the load.
A chain pulley block should also not be used for unintended side pulling unless the equipment and application specifically permit it. OSHA's crane requirements specifically caution against side pulls except where properly authorized and assessed.

10. Poor or Infrequent Maintenance
Even a well-designed chain pulley block requires appropriate inspection and maintenance.
Ignoring small warning signs can allow a relatively minor problem to develop into a significant equipment fault.
Maintenance should be based on:
• Manufacturer recommendations
• Frequency of use
• Working environment
• Load conditions
• Inspection findings
• Applicable standards and regulations
Important areas include the brake mechanism, load chain, load sheave, hooks, bearings, gears, hand chain and other operating components.
What Should You Do If a Chain Pulley Block Slips?
If a suspended load begins slipping unexpectedly, do not continue normal lifting operations to see whether the problem disappears.
The equipment should be taken out of service and assessed by a competent or appropriately qualified person according to the manufacturer's procedures.
The inspection should investigate:
1. Brake condition
2. Brake contamination
3. Brake adjustment or wear, where applicable
4. Load-chain condition
5. Load-sheave condition
6. Chain reeving
7. Hooks and suspension components
8. Signs of overloading or shock loading
9. Lubrication practices
10. Any unusual noise, resistance or movement
Do not attempt an improvised repair while a load is suspended.
If a hoist has demonstrated uncontrolled load movement, the safest approach is to remove it from service until the cause has been identified and the equipment has been properly inspected and, where necessary, repaired.
How to Prevent Chain Pulley Block Slippage
Prevention starts with using the correct equipment and maintaining it properly.
Before lifting
Check that:
• The load is within the rated capacity.
• The load is properly balanced.
• The lifting point is appropriate.
• The hook is correctly positioned.
• The load chain is free from obvious damage or twisting.
• The chain is correctly reeved.
• Hooks and latches are in suitable condition.
• The hoist operates smoothly.
• There are no obvious signs of brake problems.
During lifting
Lift smoothly and avoid:
• Shock loading
• Jerking the hand chain
• Side pulling
• Dragging loads
• Lifting loads that are stuck
• Exceeding rated capacity
The load should remain under control throughout the operation.

A chain pulley block slipping under load is a warning sign, not a normal operating condition.
The most common causes are associated with the brake system, contamination, wear, overloading, load-chain problems, incorrect chain engagement and improper lifting practices.
Because the exact cause cannot be determined from slippage alone, the hoist should be inspected rather than repeatedly operated in an attempt to diagnose the problem while loaded.