Get-a-grip: Holding the box from "slip, sliding away"

In this case, the hand is a hand holding a box from the top (Fig 1a). A free-body diagram shows the finger forces, the friction forcesand the gravity force (weight) acting on the box (Fig 1b). The sum of the friction forces must be equal to the weight or the box will fall out of the hand. Also, the sum of the finger forces must be equal to the sum thumb force. The forces required to hold the box are similar if the box is held in front of the body as shown in Fig 1c).

(a)(b)(c)

Estimate the required pinch strength to hold the box shown in Fig 1a.

It can be shown that:

Pinch force ≥ Weight / (2xCoF)

Where:

F_finger = Resultant^a finger force
F_thumb = Resultant^a finger force
F_pinch = Resultant^a finger force
F_friction = Resultant^a friction force
W = Weight (or m×g)
CoF = coefficient of friction lookup or measure
^a"Resultant force" refers to a single force vector that has the equivalent effect as the multiple forces distributed over the fingers or thumb.

F_finger = F_thumb = F_pinch
W = 2×F_friction

Pinch force calculator:

Weight = / (2× )   Calculate pinch force Required pinch force

Estimate the pinch force required for a worker packing 15N (3lb) cardboard box

Pinch force ≥ Weight / (2xCoF)

The weight is given: W = 15N
look up or measure CoF for cardboard
Previous studies by Buchholz et al. (1988) found the CoF for paper to be 0.27 for dry skin and 0.42 for moist skin. In this case, the repetitive handling of the boxes can be expected to dry the skin, CoF=0.27 is used.
15N / (2 x 0.50) = 15N (3 pounds

Fp ≥ 15N / (2 x 0.27) = 28N (6.3pounds)

Relative pinch force -- %MVC

"%MVC" (percent of maximum voluntary contraction or strength) is computed as the required force divided by strength. Although the forces on the fingers and thumb are equal, the strengths of the fingers and thumb are not. Basically, it is four against one (fingers against thumb). The thumb strength will be the limiting factor. The thumb should be used for determining if worker strength capacity is exceeded and for computing %MVC. There are some studies that have specifically examined thumb strength (Kroemer & Gienapp (1970)
Task Hand Strength Demands and Capacities