FIGURE 2. Measurements of packaging in no feedback mode.
From the following article:
The bacteriophage 
29 portal motor can package DNA against a large internal force
Douglas E. Smith, Sander J. Tans, Steven B. Smith, Shelley Grimes, Dwight L. Anderson and Carlos Bustamante
Nature 413, 748-752(18 October 2001)
doi:10.1038/35099581
The tether shortens (blue line) and the force increases (red line) as the portal motor pulls in the DNA. In this example, the motor stalled at a force of 
55 pN. Although in many measurements the linkage broke before stalling was observed, similar maximum forces could be projected from these curves. We suspect that such breakage events occur at the antibody–prohead connection. Inset, spontaneous slipping events, where the DNA comes out of the capsid, were accompanied with abrupt decreases in force.