Abstract: The lift unit comprises a bracket portion that is secured to the trailer tongue and a hinge portion that pivots on the bracket portion about a horizontal axis to raise and lower a chain for flexing and relaxing a spring bar that when flexed, serves to transfer a portion of the tongue weight to the towing vehicle. A flat rectangular member is eccentrically mounted on the side of the hinge portion to hang in the 6:00 o'clock position unless acted upon by influences other than gravity. When the hinge portion is operated to raise the chain and thereby flex the spring bar, a longer side of the rectangular member abuts and rides along the edge of a side of the bracket portion resulting in the member being swung clockwise to about the 8:00 o'clock position when full chain elevating position is reached. At this time one corner of the member drops into a notch in the bracket portion and prevents the hinge portion from being operated in the direction of chain lowering.