A capstan winch uses a mechanically powered rotating cylinder, called a capstan, to apply pulling force through a rope. When the rope is looped around the rotating capstan and tightened, friction between the rope and capstan allows the winch to apply force to pull a load. A typical capstan winch is shown in Figure 1 below.
As the rope is drawn onto the rotating capstan from the load side, the free end of the rope unwinds. A typical small capstan winch may generate several tons of pulling force. Naturally, the winch must be firmly mounted so that it will not move while pulling a heavy load. Capstan winches are often securely bolted to heavy machines or relatively immovable objects such as a concrete slab.
Recently, The Warren Group analyzed a capstan winch that moved unexpectedly. The winch was secured by steel anchors to a concrete slab. The first time the winch was used to pull a load, the anchors pulled free from the slab, and the winch tipped over onto a nearby worker’s foot, causing his foot to be severely injured. The anchors used to secure the winch to the slab were approximately 5/8 in. in diameter. Examination of the capstan winch user’s manual revealed that the manufacturer recommended 1-1/4 in. anchors. The winch was rated to pull a maximum of 6000 pounds, but the load being applied at the time of the incident is not known. Were the undersized anchors a cause of the worker’s injury?
If not held in place, the capstan winch will tend to tip over around its mounting flange when it applies force to the rope. Such a tendency to produce rotating motion is known as a moment. To avoid tipping, the mounting bolts counteract the applied moment by an expanding wedge in a drilled hole creating friction against the inside of the hole.
A simple, two-dimensional analysis of the moments and forces acting on the winch predicted that each of the two rearmost bolts would experience approximately 4000 pounds of tension. Research on concrete anchors indicated that the maximum design load for 5/8 in. concrete anchors is on the order of 3000 pounds. The 2D analysis suggested that, at full load, the winch may have exceeded the design limits of the 5/8 in. anchors. Computer-aided 3D analysis using SOLIDWORKS design software supported with the 2D finding.
Although the actual load on the capstan winch is not known, the load was likely less than the winch’s maximum. The undersized 5/8 in. anchors may have failed at less than their design load, suggesting an error in installation in addition to the demonstrated failure to install correctly sized anchors. The winch installer’s selection and installation of undersized anchors is the cause of the worker’s injury.
Jeffery H. Warren, PhD, PE, CSP, is the chief engineer and CEO at Warren specializing in mechanical, machine design and safety. His deep expertise in machine design and safety analysis makes him a frequent presenter, trainer and expert witness. In addition to investigating more than 2000 claims involving property damage and injuries related to machinery and equipment since 1987, Jeff has an undergraduate degree in Mechanical Engineering from the University of North Carolina as well as a Master of Science and a Doctorate in Mechanical Engineering from Virginia Polytechnic Institute and State University — both with machine design emphasis.