All Fingers and Thumbs
How to assess the risk of an unavoidable finger trap
Anyone who has designed anything with moving parts will know a thing or two about finger traps. We look for anywhere where a finger could accidentally be inserted into a product and try to avoid the possibility of finger insertion in places where mechanical forces might be applied that could result in injury or worst case severing of the digit. Always the preferred solution is to design out the finger trap completely.
But how do you assess injury risk where a finger trap is unavoidable?
We recently developed a product where a trap was unavoidable. This occurs when 2 components are hinged away from each other with spring-assisted closure. We needed to understand whether the minimum spring force required for the parts to reliably and repeatedly close would cause injury, if opened to the maximum possible extent, and then released onto an inserted finger. To complicate matters further the hinge is not a fixed hinge but instead comprises 2 sprung retaining cables with compound curves for mating surfaces. So the arc of travel is complex and likely to be different every time the hinge is opened.
Calibrating force against displacement
Our first step was to define ‘acceptable’ forces. What are the forces that can be applied to a finger before bruising, fracturing or breakage occurs or before permanent damage is inflicted? Surprisingly these forces do not appear to be defined in standards – we eventually discovered a couple of research papers that identified these force limits in the context of electric window mechanisms for cars. Gory reading but evidentially very credible!
For our tests to be realistic, we needed to recreate the point loading that would occur if a finger were placed in harm’s way and find a way to measure the force applied to the finger. After a lot of thought we adopted the following approach:-
We cast several sizes of simplified test ‘fingers’ in soft silicone
We calibrated the fingers by compressing them in our tensile test machine recording the force applied at increments of deflection
We built a test rig to retain the test finger at various positions within the hinged joint. Our high-speed camera, operating at 1000 frames per second, was mounted to the rig perpendicular to the rotation of the hinge to capture the point of maximum deflection of the silicone finger as the hinge was released under the force of spring-loading.
We were able to measure the deflection on the recorded high-speed footage at the point of maximum compression of the finger and extrapolate the force from the tensile test machine calibration tests.
Test rig to retain the test finger at various positions within the hinged joint
Chronos 1.4 High-Speed Camera
The slow-mo video shown at the top is slowed 35x - the real time duration of the video is 0.3 seconds. If the slow-mo video looks alarming, keep in mind that the silicone fingers are not intended to be compressively representative of real fingers. We went for the softest silicone to maximise deformation so that our measurements could be as accurate as possible.
Sorry if it makes you wince!