On a sunny day in New Zealand, an incident occurred while operating a swing yarder at a landing, resulting in a tower collapse.
The operator had over 15 years of experience on this type of yarder.
The yarder was being turned on the same landing for the next setting. Guy cables had been slackened (not connected to stumps) and had big bellies. The tower was positioned at 70 ft. The main rope was connected to the carriage with the clamps on. The carriage was located in front of the yarder and no other working cables were connected at the time of the incident.
The crew was running straw line ready for the yarder to pick up after it had been repositioned and the guy cables re-tensioned. There were no other persons working on the landing or behind the yarder at the time.
The yarder driver was about to push the lever that would place the tower in its correct pulling position, when he saw the carriage moving toward the yarder. There was a bang and the tower fell to the ground behind the yarder.
While repositioning the yarder, the stabilizer ram failed resulting in the pole collapsing backwards.
Unsafe Act or Condition:
The incident investigation identified several contributing factors.
- Failure of the stabilizer ram - there was a failure with the connection of the piston to the ram. The piston is normally done up very tight and held in place by a set screw and sometimes Loctite (or similar). In this case, the stabilizer ram didn’t have set screws installed. This allowed the stabilizer ram to pull apart after the piston unscrewed from the end of the ram. The set screws are not visible, so without taking the ram apart there would be no way to know they were installed. Standard annual tower inspections would not pick this up.
- A snap guy (front guy cable) was not in use as a safety precaution. The snap guy would not have prevented the stabilizer rams from failing but would have reduced the risk of the tower falling.
Recommendations for Correction:
- In this scenario, it is best practice to use a snap guy cable to counter forces from behind the yarder. It is recommended that this is put in place at the same time as the guy cables are connected and before lifting/tensioning. A snap guy can be any of the three working cables (Sky, Main or Tail, depending on the system being used.) This process should be carried out at 50 ft. before any of the guy cables are tensioned.
- It is not recommended that the yarder is moved whilst the tower is at 70 ft. This could create a pendulum effect and make the yarder unstable. This will create extra weight and stress on the stabilizer ram and other parts of the tower. Follow the manufacturer’s recommendations for shifting machine.
- Install a suitable strength safety strop (chain) on the stabilizer ram. This will help prevent the tower from falling over in the event of stabilizer ram failure.
- Do not assume that the area directly behind a yarder is a safe zone. In this case, there were log stacks behind the yarder. Often, we observe crew vehicles, safety zones, log decks and crew break huts behind the yarder, as this is assumed to be a safe zone.
- Mark and monitor the length of the exposed hydraulic ram to confirm separation is not occurring.
- Ensure that only competent and qualified experts perform maintenance / repair work on machinery, especially safety critical components.
- The tower should be in the travel position or at least lowered to 50 ft. to move and rig. This greatly increases stability and reduces the chance of failure (as per manufacturer’s recommendations)
- A safety strop (chain) could have been fitted on the stabilizer ram. While a strop would not have prevented the ram from failing, it would have prevented the tower from falling over when the ram failed.
Figure 1: Sketch of stabilizer ram failure and yarding tower collapse
Vickie Swanton, FRA Western Region Manager