An E.O.T. crane stands for Electric Overhead Travelling crane. This is used for handling & moving a maximum specified weight of the components called capacity of the crane within a specified area. The crane can be operated manually or by electric power.
Types of Electric Overhead Cranes
There are various types of overhead cranes with many being highly specialized, but the great majority of installations fall into one of three categories:
- Top running single girder bridge cranes
- Top running double girder bridge cranes
- Under-running single girder bridge cranes
Electric Overhead Travelling (EOT) Cranes come in various types:
- Single girder cranes – The crane consists of a single bridge girder supported on two end trucks. It has a trolley hoist mechanism that runs on the bottom flange of the bridge girder.
- Double Girder Bridge Cranes – The crane consists of two bridge girders supported on two end trucks. The trolley runs on rails on the top of the bridge girders.
- Gantry Cranes – These cranes are essentially the same as the regular overhead cranes except that the bridge for carrying the trolley or trolleys is rigidly supported on two or more legs running on fixed rails or other runway.
- Monorail – For some applications such as production assembly line or service line, only a trolley hoist is required. The hoisting mechanism is similar to a single girder crane with a difference that the crane doesn’t have a movable bridge and the hoisting trolley runs on a fixed girder. Monorail beams are usually I-beams (tapered beam flanges).
Which Crane should you choose – Single Girder or Double Girder?
A common misconception is that double girder cranes are more durable! Per the industry standards (CMMA/DIN/FEM), both single and double girder cranes are equally rigid, strong and durable. This is because single girder cranes use much stronger girders than double girder cranes. The difference between single and double girder cranes is the effective lifting height. Generally, double girder cranes provide better lifting height. Single girder cranes cost less in many ways, only one cross girder is required, trolley is simpler, installation is quicker and runway beams cost less due to the lighter crane dead weight. The building costs are also lower.
The advantages and limitations of Single / double girder cranes are as follows:
Single Girder Cranes
- Single girder bridge cranes generally have a maximum span between 20 and 50 feet with a maximum lift of 15-50 feet.
- They can handle 1-15 tonnes with bridge speeds approaching a maximum of 200 feet per minute (fpm), trolley speeds of approximately 100 fpm, and hoist speeds ranging from 10-60 fpm www.PDHcenter.com PDH Course M245 www.PDHonline.org
- They are candidates for light to moderate service and are cost effective for use as a standby (infrequently used) crane.
- Single girder cranes reduce the total crane cost on crane components, runway structure and building.
Double Girder Cranes
- Double girder cranes are faster, with maximum bridge speeds, trolley speeds and hoist speeds approaching 350 fpm, 150 fpm, and 60 fpm, respectively.
- They are useful cranes for a variety of usage levels ranging from infrequent, intermittent use to continuous severe service. They can lift up to 100 tons.
- These can be utilized at any capacity where extremely high hook lift is required because the hook can be pulled up between the girders.
- They are also highly suitable where the crane needs to be fitted with walkways, crane lights, cabs, magnet cable reels or other special equipment.
An EOT crane consists of two distinct parts
The Bridge consists of two main girders fixed at their ends and connected to another structural component called the end carriages. In the two end carriages are mounted the main runners or wheels (four or more) which provide the longitudinal motion to the main bridge along the length of the workshop. The motion of the bridge is derived from an electric motor which is geared to a shaft running across the full span of the bridge and further geared to a wheel at each end. In some design separate motors may be fitted at each corner of the main bridge. The wheels run on two heavy rails fixed above the floor level along the length of the shop on two girders, called gantry girder.
The Crab consists of the hoisting machinery mounted on a frame, which is in turn mounted on at least four wheels and fitted with suitable machinery for traversing the crab to and fro across the main girders of the crane bridge. Thus the load hook has three separate motions, these being the hoisting, cross traverse of the crab, and longitudinal travel of the whole crane. Each motion is controlled independently of the other motions by separate controllers situated in a control cage or in a suitable position for controlling from the floor by pendent chains.
Quality of EOT cranes:
- High efficiency: Designed for high speed operation to ensure increased productivity, to minimize losses and to increase overall efficiency.
- Smooth operation is ensured by:
- Gears having precision hobbed teeth which are subjected to heat treatment.
- Machined components checked by limit gauges.
- Antifriction bearings on all high speed rotating parts.
- Ease of Maintenance
- Easy accessibility of parts requiring maintenance.
- Minimum number of maintenance parts is used in the crane.
- Easy to remove and replace wearing parts.