Late answer but hope it is relevant: There are several outdoor sliding gate
door opener/closer acuators being made, which consist of a winch drum style
device with a steel cable anchored at both ends on the gate making one loop
on the drum. End stops are sensed by sensors. The motor is typically
capacitor run AC. The gates weigh in easily at 3 tons (some of them), and
some are longer than 3m and have their own wheels to support them, or run o=
n
a bottom rail. I see these are hard to find online, ask a gate maker near
you. Industrial more likely than residential.

On the contrary, the toothed rack type gate actuator is popular (but has
alignment problems with a standard steel rack - the cable kind is very
inseisitve to alignment problems). I assume something like this:
http://www.alibaba.com/product-detail/AC-MN370-Sliding-gate-opener_21276996=
5.html
can be installed on the carriage and made to work against a tensioned fixed
and roller-pressed toothed belt of suitable pitch. This is the only option
which requires no greasing/maintenance and likely runs smooth. The cable
type can run very smoothly if the cable is chosen appropriately. Stranded
steel cable does not run smooth on a drum most of the time and corrosion is
a serious problem since greasing cannot be done (won't grip on the drum).

This model uses a chain to move the gate, also an option (especially if you
like scary haunted castle noises unless very well maintained):
http://www.alekoproducts.com/ALEKO-AC1500-Sliding-Gate-Opener-p/ac1500-ap.h=
tm

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