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Sunday, 14 December 2014

EOT CRANE MAINTENANCE

Electric overhead travelling crane

During our discussion in E.O.T crane: Part 1, we have discussed the fundamentals of EOT crane which could be seen in my previous post.

After understanding this blog we will have detailed information about various aspects of electric overhead travelling crane,

So, let us start without wasting time ...

Basic component of E.O.T crane

Before going in deep discussion, first we must understand various components of electric overhead travelling crane.

E.O.T crane: components



Let us understand about the various terms that we must have to mention for specifying an electric overhead travelling crane.  


Let us consider each term that we used to use for specifying our requirements during selecting an ideal E.O.T crane for meeting our need.

Capacity of Crane

Capacity of crane is normally mentioned, by the manufacturer, over the name plate of crane. It indicates the safe working load for which crane or its hoisting mechanism is designed. Let us think if safe working load 25 ton is mentioned over the name plate of a electric overhead travelling crane , its mean we must lift the load 25 Ton or lower than 25 ton but we must not go beyond safe working load.

Lift height 

Crane lift height indicates the distance between lower and upper level of snatch block or we may say that by how much height a crane can lift the load.
Lift height = Distance between beam and floor – height of crane hoist.

Runway height 

It is basically the distance between top of rail and grade rail.

Clearance

It is basically vertical distance between crane girder bottom of rail and grade rail.

Crane Span 

Crane span is basically the distance which is measured horizontally between the runways rails over which crane will travel.

Runway length 

It is basically the longitudinal length of runway which is measured parallel to the building length.

Electrical requirement 

According to US requirement for E.O.T Crane, we have noted the desired power supply which is given via conductor bar and power supply will be 480 volt, 60 Hertz and 3-phase.

When we select one E.O.T crane, in that condition apart from above need, we also mention some specific need based on the application of E.O.T crane. We have mentioned some examples below based on the professional working and maintenance experience of E.O.T crane.

1.    Requirement of second hoisting mechanism?

I have observed E.O.T cranes those are used in the field of steel industries for various activities such as for tapping or pouring and such cranes use additional hoisting mechanism and this additional hoisting mechanism will be called as auxiliary hoist.

We were using one E.O.T crane in one steel industry for tapping and pouring liquid hot metal in furnace or in ladle.

We specify the capacity of such E.O.T crane as mentioned following

100/60/25 Ton

It indicates that primary hoisting mechanism will have 100 Ton capacity and there are two auxiliary hoisting mechanisms whose capacity will be 60 and 25 Ton respectively.

2.    Condition of environment where we will use this crane.

3.    Use of anti collision device for avoiding the collision between two cranes if more than two or two cranes are traveling on same bay.

4.    Requirement for lightening, lamps and any other important 

5.    Need of catwalk for maintenance access.

Question: What is rated capacity of crane?

Rated Capacity of crane is basically the load that could be lifted easily by the hoisting mechanism of crane.

It is basically maximum working load which is suspended under snatch block. Weight of snatch block and weight of wire rope will not be included in Rated load of crane.

Question: What is test load of crane?

According to OSHA and ASME, crane load testing will be carried out at 125 % of rated capacity of crane.

Crane Duty Group

By considering crane duty group we will come to know that the particular crane is designed for which type of duty such as for light duty, medium duty, high duty or very heavy duty. 

It is very necessary to understand the crane duty group as we can't use a crane into continuous heavy duty work even crane is designed for light duty, hence for safety reason we must understand the crane duty group.

There are several organization and I am sure we heard the name like ISO, HMI, FEM and CMAA. These organizations have their own standard and classifications for crane duty group but these all organization use the similar principle and facts for classifying the crane duty group.

Let us understand about the crane duty group and what it is used for 

CMAA Crane classification

According to the CMAA specification”70’’ for crane classification, there will be six different classifications as we have mentioned below and these classifications are dependent on duty cycle.

CMAA Class

Description

Details

C
Moderate service
Crane under this classification will be operated approx 50 % of the rated capacity and with 8 to 10 lift per hour.
These types of cranes are used in machine shops
D
Heavy service
Crane under this classification will be operated approx 50 % of the rated capacity but continuously throughout the working period and with 10 to 20 lift per hour.
These types of cranes are used in heavy machine shops, container yard , fabricating unit , foundries unit and  steel warehouses
E
Severe service
Crane under this classification will be operated approx the rated capacity throughout the working period and with more than 20 lift per hour.
These types of cranes are used in container handling ,cement mills, scrap yard, fertilizer plant and lumber mills
F
Continuous Severe
Service
Crane under this classification will be operated the rated capacity throughout the life of the crane and this type of crane will be used for critical work tasks with higher degree of reliability

AISE standard for classification of crane duty cycle

We normally prefer the classifications given by AISE Technical Report No. 6-Specifications for Electric Overhead Travelling Cranes for Steel Mill Service.

AISE also provided standard for classifying the crane duty type and these classifications are as mentioned below.

Sr. No

Service Class

Cycles

1
Class 1(N1)
 Less than 100,000
2
Class 2(N2)
100,000 to 500,000
3
Class 3(N3)
 500,000 to 2,000,000
4
Class 4(N4)
More than 2,000,000

Further AISE provided the various load classes

Sr. No

Load Class

Cycles

1
L1
Cranes under this standard will lift the rated load rarely  and normally lift very light loads
2
L2
Cranes under this standard will lift the rated load rarely  and normally lift 1/3 of the rated capacity
3
L3
 Cranes under this standard will lift the rated load frequently and normally lift 1/3 to 2/3 of the rated capacity
4
L4
Cranes under this standard will lift the rated load continuously

Hence on the basis of load cycle and load classes, CMAA had provided one chart for easy understanding the class of E.O.T cranes.

Load Classes

Load  Cycles

N1
N2
N3
N4
L1
A
B
C
D
L2
B
C
D
E
L3
C
D
E
 F
L4
D
E
F
F

Average daily operating time

T= 2*H*N*T/ (V*60)

Where,

H = Average hoisting height (ft or meter)
N= Number of working Cycles/ hour
T= Working time in one day (Hours)
V= Speed of hoisting (m/min)

Keep Reading ....... 

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