Crane Warning Systems

Crane Safety Systems FAQ

What is the difference between a load indicator and an LMI (Load Moment Indicator)?

These are the three most misused and confused terms in the crane indicator business. A crane load indicator is a cab mounted display connected to various types of load sensors that show the crane operator the weight they are lifting. The load indicator can have other options to monitor more additional information such and anti-two-block, boom angle, length, and radius, but it does not monitor the crane's capacity. So it does not have the crane's load chart programmed into memory and does not calculate the safe working load or warn of an overload unless the operator has manually set a load limit. A load moment indicator or LMI and a rated capacity indicator or RCI, show the operator the load, capacity, boom angle, length, radius, and usually anti-two-block. The LMI and RCI achieve the same load reading but go about it by different methods. So the main difference between a load indicator and an LMI or RCI is that only the LMI and RCI monitor the cranes capacity and warn of overload.

What is the difference between an RCI and LMI?

The difference is in how the force/weight is measured. An RCI or rated capacity indicator uses load sensors that measure the force directly on the hoist line. For example, a dynamometer, also known as a "line rider" uses a load pin in a sheave that the hoist line exerts a force on. The pin then outputs an electronic signal, which can be translated into the weight reading. The other common type of load sensor used with an RCI is a load link that measures the force being pulled on the hoist line. These can be installed on the "dead-end" where the hoist line is secured to the boom or when using a wireless load link installed above the hook block. Both methods involve measuring the force directly off the hoist line that is lifting the weight.
The LMI or load moment indicator uses sensors that measure the force on the boom, not directly on the hoist line to calculate the weight being lifted. An LMI can use load pins that measure the force on the pendant line on a lattice boom crane or pressure transducers that measure the rod and bore side pressure of a boom lift cylinder. LMI systems use physics to arrive at the weight based on the force on the boom along with the boom angle and length.

What crane indicator do I need for my crane?

A crane load indicator will be less expensive than a load moment indicator (LMI) or rated capacity indicator (RCI), but it also provides much less information. Knowing the weight, you are lifting is critical for working safely and keeping your crane from being overloaded, so a load indicator is helpful for that. However, a load indicator still leaves it up to the crane operator to know how much they can lift at each change in their boom angle or radius. The LMI and RCI calculate all that for the operator and show it to them in real-time with the actual numbers and easy to read bar graphic that is green when you are under capacity, yellow as you get near the limit, then red at overload. This guy may have saved some money on a load indicator, so he knew what the car weighed, but an LMI or RCI would have let him know it was more than he should lift.

What is an anti two block warning system?

The anti-two-block warning system or indicator is also known as an A2B or ATB indicator for short. This is the minimum crane safety indicator all cranes should have. It warns the operator when the hook block is getting too close to the boom tip. A2B warning systems come in wireless and wired versions. The only difference is one requires a cable to connect from the display to the A2B switch at the boom tip and the other uses a wireless transmission for the connection. The ATB warning system uses a switch at the tip of the boom with a weight on a variable length of chain hanging down the hoist line. When the hook block comes up it lifts the weight which closes the ATB switch and sets of the ATB alarm in the cab. This alerts the operator and anyone in the area that the crane is close to having a collision with the hook at the boom tip. This is a picture of a wired ATB switch and the weight and chain hanging from it which holds the switch open. In the picture the block is getting close to lifting the weight which will let the ATB switch close and set off the two-block alarm. This is what the R140 wired ATB warning alarm looks like.