The connection between your tow vehicle and your trailer serves a number of purposes. The primary one is to provide a solid mechanical connection between the two. Another purpose is safety in case of failure or other problem. A third purpose is to provide control of motions, both up and down as well as side to side, at the hitch point.
All parts of the hitch must be properly rated for their intended use. There are significant forces involved and the materials and structural design of each and every part must be sufficient to handle all expected contingencies.
The receiver hitch is the part that bolts to the frame of the tow vehicle. It ends in a square tube. The ball mount slides into this tube. Both the receiver and the ball mount have a hole that allows a pin to slide through and hold them in place. The receiver is not only designed to hold the trailer hitch off the ground, it is also designed to transfer weight from the hitch forward towards the front wheels. This weight transfer is done by other hitch parts that twist the receiver to pick the rear end of the tow vehicle and push down on the front end. This twist or torque is why the receiver has to be very strong and well connected to the vehicle frame.
Note that receivers will have different ratings depending upon whether they are used with or without a load leveling hitch. When a hitch with spring bars and an appropriate ball mount applies torque, all of the receiver mounting bolts share the hitch load so it can handle a larger load. If the trailer tongue is just dropped on the ball without load leveling apparatus, then the rear bolts on the receiver have all of the tongue weight pushing down while the front bolts have it pushing up so the receiver can't handle as much as it could with a load leveling system.
The ball mount can be simply a mount for a trailer ball on the square pipe that fits into the receiver or it can be a much more complex mounting point that has connections for spring bars, anti sway dampers, or special pivot point geometry devices (e.g. the Hensley hitch). This is usually the heaviest hitch component and may way up to a hundred pounds. The ball mount shown here is for the Equal-i-zer hitch. Note that slot for the top bolt that allows adjusting the ball angle. The washers on a bolt at the bottom of the picture are used to adjust this angle. The ball is bolted through the hole at the top top right of the ball mount in the picture.
The shank slips into the receiver and provides a mounting point for the ball mount assembly. The photo shows an adjustable shank that allows the ball mount assembly to be moved up or down by using different sets of holes in its vertical part. The leg with only one hole is the one that slides into the receiver. The hole is to accommodate the hitch pin that keeps if from sliding out after it has been installed in the receiver.
The hitch pin goes through holes in the receiver and shank to keep them together and is secured with a clip to keep it from falling out.
The trailer ball is the tow vehicle part of the ball and socket system that couples the trailer to the tow vehicle in a manner that allows a lot of both vertical and horizontal movement. The main trailer ball may also have a smaller ball at the side to allow connecting a sway control device.
The coupler is the trailer part of the ball and socket system. It sits down over the trailer ball and has a latch to keep it seated.
Are a backup system whose purpose is to keep the trailer coupler off the road in the event of a failure of the hitch. Chains should be loose enough to allow the hitch to operate normally but short enough to keep the hitch parts off the road in case the ball breaks or comes loose or there is some other catastrophic hitch failure.
There is an electrical umbilical cord between the trailer and the tow vehicle. It provides circuits for running lights, brake lights, and turn signals as well as brake control and battery charging.
The breakaway switch is an emergency trailer brake activation switch. The pin on the switch is connected to a cable that is attached to the tow vehicle. The idea is that if the trailer breaks loose, the breakaway switch will activate so that the whole rig will be brought to a safe stop. There is some controversy about when this should be configured to activate. Should it be short enough so that it will apply the brakes if the safety chains are called into play or should it be longer so it only applies emergency braking if the trailer totally disconnects from the tow vehicle?
Spring bars are steel bars that attach to the hitch ball assembly and run alongside (usually just below) the A Frame and are then attached to the A Frame near the trailer wall with chains or brackets. At the hitch ball assembly the spring bars need to be able to rotate back and forth but not up and down.
The front frame members of the trailer that connect the frame rails of the trailer to the coupler are called an A Frame. Propane tanks are usually set in a bracket attached to this A Frame and a hitch jack is mounted towards the front where the frame comes together at the coupler.
There are a number of mechanisms that are used to dampen the trailer's impact on the behavior of the tow vehicle. See Understanding Sway and Sway Control for more about this topic. The most common method is a brake bar mounted between the A-Frame and a small ball a couple of inches to the side of the main trailer ball. This brake bar has an adjustment for the amount of friction is applies resisting the rotation of the trailer about the hitch.
See the page on hitch setup to learn about how these parts and pieces fit together to keep your trailer and tow vehicle connected for a comfortable and save journey.
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U.S. Department of Transportation - National Highway Traffic Safety Administration (NHTSA) - Towing A Trailer, Being Equipped for Safety - Title Page and Table of Contents - DOT HS
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