Imagine this: the operator pressed the load lift button on the control panel and became distracted. The hook suspension is inexorably moving upward. If left unchecked, it will crash into the drum, break the rope, or burn out the electric motor. One small component—the limit switch—can help avoid this disaster.
In Bulgarian hoists (T, MT series, etc.), limit switches are the primary safety devices. They automatically cut off power to the motors when they reach the maximum or minimum limits. Let's explore how they work and take a closer look at the most popular models on the market: the KI-62, KI-G1, KI-G11, and KI-PDT.
How does it work? A quick primer
In a classic wire rope hoist, the limit switch for the lifting mechanism is hidden under the cover of the electric motor's terminal box. Its lever is connected to a special rod located along the drum. Limit rings (called "crackers") are attached to this rod.
When the rope guide reaches its upper or lower limit, it rests against the "cracker" and moves the rod. The rod pulls the limit switch lever, which opens the control circuit (low-current contacts). The motor stops immediately. However, reverse movement remains possible.
Overview of Popular Limit Switch Models
Bulgarian limit switches are valued for their reliability. Their housings are cast from a special fireproof plastic that can withstand extreme temperatures up to +300°C.
Here are the four main models you'll definitely encounter when servicing hoists:
1. KI-G11: For "junior" hoists
This is a standard limit switch, installed on lightweight rope hoists with lifting capacities of 0.5 and 1 ton.
Feature: It has a special lever mount (with a hole) for connecting to the traction rod.
Purpose: Limits the upward and downward movement of the hook. A reliable, simple, and very common option for small workshop hoists.
2. KI-G1: The Workhorse
The older brother of the G11. Externally, they are very similar, but the KI-G1 is designed for more demanding equipment. It is installed on hoists with lifting capacities from 2 to 10 tons.
Special feature: Unlike the G11, this one has a special groove for the fork (rather than just a hole) on the lever, due to the different design of the rod on larger hoists.
Purpose: Locks the lifting mechanism in both directions. Withstands harsh operating conditions in heavy-duty production environments.
3. KI-62: Universal for Chain Hoists
While the KI-G series is more commonly found on rope machines, the KI-62 is a frequent guest on traveling chain hoists (with a lifting capacity of 0.5 tons and above) and in various crane mechanisms.
Feature: It differs in its housing shape and actuation kinematics. Like its counterparts, it has contacts for lowering and lifting, operating in the control circuits of magnetic starters.
4. KI-PDT: Two-Stage Protection
The abbreviation PDT stands for "forced action, two-stage." This is the most advanced and safe limit switch in the line (including modifications KI-PDT 2, 22, 23, 24, etc.).
Feature: It features a dual level of protection (two stages).
How it works: If, upon reaching the limit position, the first pair of contacts (the working ones) sticks or fails for some reason, the rod continues to press on the lever and activates the second stage. The second stage completely and urgently de-energizes the system (often by breaking the common circuit), preventing an accident. Such switches are installed in facilities with increased safety requirements.
Operating Tips
Bulgarian-made limit switches can last for years, but they require attention:
Check the "crackers" regularly: Vibration can cause the limit rings on the rod to shift. If this happens, the limit switch will operate too late.
Keep it clean: Dust and oil inside the terminal box can cause poor contacts or mechanism sticking.
Bottom Line: A limit switch is inexpensive, but it can save hundreds of thousands of rubles in equipment. Select a spare part strictly based on the lifting capacity of your hoist and don't skimp on safety!
