Dynamic stability of under hoist transmission jacks
Abstract Under hoist transmission jacks are indispensable tools for vehicle service and maintenance. They are critical due to their high height/base width ratio, large carried weights and size, poor visibility. This work moves from the analysis of an actual accident where a trans-jack overturned; this event is one of manual handling injuries, usually classified as being struck by an object and or equipment in statistical reports. Requirements to achieve a good stability are here studied and defined; more in detail, the dynamic stability of a traveling transmission jack changing its speed has been studied as well as the stability of a transmission jack suddenly stopped due to wheel impingement since, up to now, there is not specific directive addressing these conditions. Results have demonstrated that the instability while translating is not likely to take place for usual geometries; it could become more critical for higher center of mass positions, lower points of application of the force (for example when using feet instead of hands), and for lower carried weights. On the contrary, the instability due to a sudden stop represents a real possibility; it becomes more critical for higher center of mass heights, higher pushing forces, or higher points of application of the force. The introduction of a translation speed limit is therefore strongly recommended. Other operative advice are that higher points of application of the pushing/pulling force are convenient when changing the traveling speed of a transmission jack, while lower handles position should be used while moving at constant speed.