Understanding and Fixing my Gate
Texas introduced us to cowboy hats and electric gates, in our case a a Nice Apollo 1500LA. It’s a long-running linear actuator swing gate: simple enough to be something I can repair, but just complicated enough to be difficult. The whole gate is really two different parts: a 636 control board and a 816 actuator. In the past, I documented how I connected our gate to Alexa through a WiFi connection in Gate Automation. This post is about how our gate works and how to fix it.
Our gate started opening past the limit switch and now I have to fix the 816 actuator, it’s a 12-volt DC motor that turns a pinon and spur gear which drives the central Acme lead screw. I had just fixed the broken spur gear and the old motor that lost much of its usable torque under load. After a couple of successful months after I replaced the motor and the spur gear, the limit screw broke and the gate only stopped at at absolute min extension. Slide below to see all these parts and how they work.
I opened up the housing to find that the A2019 limit screwwas broken and the key was broken as well. The A2019 limit screw threads directly the end of theACME lead screw so they move together. The illustration below shows how this works. A travellercarrying amagnetrides on the limit screw‘s thread. Two guide bolts keep the magnet from turning.
As the arm extends or retracts, the traveller walks slowly along the tower. Its position mirrors the gate. At each end of its travel sits a proximity switch on a limit block. When the magnet reaches a switch, the switch trips and tells the control board to stop the motor, one switch for fully open and one for fully closed.
So how does a magnet generate a signal to the controller board? A magnet induces an electric field. As the magnet approaches, the field strength \(B\) at the switch climbs steeply, roughly as \(1/d^3\), but continuously. My guess is that the gate has a Hall-effect sensor where current through a thin semiconductor is pushed sideways by the field, producing a tiny voltage proportional to \(B\). On its own that’s still analogue. An on-chip Schmitt trigger (a comparator with two thresholds, on above one and off below a lower one) flips its output transistor fully on or off.
Another interesting thing: the lead screw moves about 5 tpi (threads per inch) or 0.20–0.22 \(in\) per turn to fully open the gate, but the limit screw moves about \( \frac{1}{8}^{\text{th}} \) that distance. In all, whole 2 ft of arm travel is shrunk into about 3 in of traveller movement. Simply, the screw goes a distance \(L\) which is each spiral thread winding around the screw \(n_{\text{starts}}\) \(\times\) the pitch \(p\) of each screw.
In this setup, both screws turn together. The limit screw is fixed to the end of the lead screw, so after a common set of \( N\) revolutions each screw goes a total length \(s\) of:
For the full stroke the revolutions are the same,
So the lead screw has 8x the thread density and goes 8x shorter in length.
The geometry of the setup can teach us a lot about how stuff works. The lead screw pushes the rod out several feet, but it pushes only about 4.5 ft from the hinge, so a short push near the pivot becomes a long sweep at the gate’s far end. Two feet becomes 25 feet: about 20 in of stroke swings a 16 ft gate through 90°, and its tip travels about 25 ft, a quarter of a 100 ft circle. That is about 15× multiplication: the actuator’s average lever arm is about 12.7 in, compared with the tip’s 192 in, so each inch of push moves the tip about 15 in.
But nothing comes for free. The trade-off is force: holding the gate against a push at its tip takes about 15 times that force at the rod, which is why the motor’s torque is multiplied first by the gears and the ACME screw. All together, the whole chain: about 840 motor turns leads to 114 screw turns leads to 24 in of arm travel which is about 25 ft at the gate tip.
The most interesting thing for me is the offset arm. Without it the lead screw would just pull on the hinge and not push out. The bigger the offset arm, the more force transfers to the tip of the gate. How big should we make this offset? The manual recommends 6 \(\text{in}\) which has enough lever arm to break the gate away from closed, while keeping the stroke inside the actuator. More offset means a longer stroke for 90°. The actuator swings through a bigger angle, and past a point it runs out of its 24 in, which is the demo’s warning.
| Arm offset | Lever arm at start | Hinge torque | Force at tip |
|---|---|---|---|
| 0 in | 0 in | 0 lb·ft | 0 lbf (stuck) |
| 2 in | ≈ 2.5 in | ≈ 73 lb·ft | ≈ 4.6 lbf |
| 4 in | ≈ 5.0 in | ≈ 146 lb·ft | ≈ 9 lbf |
| 6 in (manual) | ≈ 7.5 in | ≈ 219 lb·ft | ≈ 14 lbf |
| 9 in | ≈ 11.4 in | ≈ 333 lb·ft | ≈ 21 lbf |
The experiment below brings all this together. Move out the pivot arm to see the effect on the force.
Now that we know how everything works, I actually have to fix this gate and put the washers and spur gear together.
The washers in the A2087 include two needle thrust bearings. Every time the actuator pushes or pulls the gate, the gate pushes back along the lead screw, trying to shove the whole screw lengthwise out of the housing with hundreds of pounds of force. A thrust bearing takes that end-to-end load while still letting the screw spin freely. Each bearing is a sandwich. In the middle is a flat steel cage holding about twenty tiny hardened needle rollers, arranged like the spokes of a wheel. On either side is a hardened, polished race: the thin washers, plus one thicker race for the screw’s shoulder. The needles roll between the races instead of sliding, so the friction stays low even under heavy load. The races matter as much as the rollers, because they give the needles a hard, smooth track; without them, the needles would quickly dig grooves into the softer housing and gear. When a gate is making bad sounds opening and closing, it’s generally overlooked parts like this.
The biggest problem for me is that little piece on the bottom right. Known a key, it’s the smallest part that does the most work: ensuring the torque is transfered from the rotating shaft to the spur gear which sits on a round, smooth section of the shaft. On its own it would just spin in place, or slip under load. To make this work, a small slot is milled lengthwise into the shaft (the keyseat), and a matching slot runs through the gear’s bore (the keyway). The square key sits half in each, bridging the two parts. All of the motor’s torque passes through that little 3/32 \(\text{in}\) bar. The Flexloc nut only stops the gear sliding off the end; it isn’t what makes the gear drive the screw.
As an engineer, I look at parts like this carefully. The key is like the derailleur hanger on your bike, strong enough to work, but weaker than the expensive parts around it. If the gate jams hard, it tends to shear the cheap key, saving a costly stripped gear or a twisted screw.
The final experiment is the diagram that matters for my current fix. Explode the assembly to see how the key ties everything together.
Hopefully this post gave you new appreciation for the beauty and thought that goes into the simple stuff around you. Today the combination of AI, youtube and Amazon/e-commerce let you understand, learn and find any part you need. These are amazing days to be a builder.
