Genie pre-2011 screw drive
carriage assembly

General thoughts on screw drive three-piece rail systems:

Some screw drive systems use a three piece rail, which is more compact, and easier to sell (and carry out) at the big box stores. That means there is a joint where the rail sections are connected. Probably the time where this is important, is when you have a screw drive machine. This is because the screw is also in three pieces. Where the screw sections connect is a joint of some kind. On Genie machines, and I suppose, on any other three-piece rail system, wherever there is a connection, it creates a gap in the screw threads of an inch or so. Genie machines have what is called a clip and collar connection. The collar slides onto the joint in one direction so the scarfed ends can be slipped together. The collar is then slid back the other way to lock the joint together and then the clip is snapped over the bare shaft to keep the collar from sliding back and releasing the joint. Makes a clever connection that holds up under some pretty good stresses. Unfortunately, this connections also means there is no screw threads in that area for the carriage assembly rack to ride in.

Chamberlain replaceable
screw drive rack

The rack is made long enough to bridge that gap with no noticable disruption so long as the teeth on the rack that engage the threads on the screw shaft are complete and relatively unworn. Normal wear and tear, and the occasional jerk or shudder caused by any number of things, may create a failure mode. This could include lack of lubrication on the screw or door rollers, the door hitting some unexpected object during its travels, the torsion springs being broken or not wound properly to balance the door, etc. The teeth on the rack may wear to the point they not longer grab the screw securely. The teeth may shear off due to excessive wear or impact. Any of these things may preclude the rack being able to ride over the 'gap' created by the rail connection. The result is the door hangs up at that point, usually while raising it. Many people think that the cause is a broken screw or bad motor, or some other part failure. If you call 888-346-3173, the helpful folks at garagedoorsupplyco.com will help you determine the root cause so you fix the problem right the first time.

Genie late model
carriage assembly

Fortunately, the fix is a simple and inexpensive one. Replacing the carriage assembly, or the rack (on Chamberlain openers) will get you back in business for pocket change (compared to the cost of replacing the other, more expensive parts that have just been protected by that sacrificial part).

Sometimes those dratted engineers do some good work, don't they!

Personal Gripe Moment: Incidentally, my pet peeve is engineers who go directly from high school to college and graduate with a shiny new engineering degree without any actual work experience at all. These 4 year wonders have no clue whatever about how things go together, and what it's like to repair an over-engineered but under-designed piece of equipment. IMHO, every aspiring engineer-to-be should be required to do hands-on work in their career field before they are allowed to graduate.