Question 1:
The three advantages of blocking and lifting over the front idler tumblers are:

a. When the front idler tumblers are blocked, the fulcrum or tipping point is moved from the centerline of the front rollers to the centerline of the front idler tumblers. This increases the stability of the crane because the lever arm from the composite center of gravity of the crane to the new fulcrum point is now longer, as much as 2' to 3' for some cranes.
This is similar to a teeter totter where a small child sitting near the end of the board, balances a young man sitting on the other side, but nearer to the fulcrum point. If the small child moves to the very end of the board, he can now balance a larger man sitting at the same distance from the fulcrum point as the young man was sitting.
Lifts should be made over the front of the crawlers with the idler tumblers blocked; not to get increased lifting capacity, but to increase the safety factor of the lift.

b. The longer lever arm also reduces the soil bearing pressure under the toes of the tracks, because there is now a longer portion of the track bearing on the soil.
This is true, only if the crane is set up level and the front idler tumblers are blocked, using the manufacturer's recommended thickness of steel plate or hard wood. If a thicker plate is used than recommended or if the crane is set "up hill" with the track toes high, there is a danger that the front idler tumblers will become point loaded and, therefore, will have extremely high soil bearing pressures.

c. Increased stability is also attained by lifting over the front tumblers because the weight of the drive sprocket and chains or drive motors that connect to the drive tumblers provide additional counterweight.
Since strength factors are generally closely related to a crane's stability, blocking the idler tumblers should never be used to attempt to increase a cranes capacity without written approval of the crane manufacturer.


Question 2:
To correctly block the idler tumblers, steel plate or hard wood blocking should be placed at location "b", and in a thickness recommended by the crane manufacturer.

The thickness will range from about 1/2" to 1-3/8", depending on the crane manufacturer. The blocking width should be a maximum of 24" so it will not extend back under the front rollers. The length of the blocking should be equal to the width of each tread + 12" (for a 6" extension either side of the track).

The following procedure should be used to block the front tumblers:

a. Lay crane mats transverse to the tracks. See Quiz No. 14 for crane mat layout.
b. Walk the crane upon the mats until it is within the set radius for the lift.
c. Set the travel locks on the crane.
d. Walk the crane back against the travel locks.
e. Check the radius to make sure the crane is still within the set radius.
f. Mark the location of the centerline of the front idler tumblers on the crane mats.
g. Walk the crane slightly forward until the travel locks can be disengaged.
h. Walk the crane backward about 5 feet.
i. Place the steel plate or hard wood blocking on the crane mats. Center the blocking so that there is 12" either side of the idler tumbler centerline mark on the crane mats.
j. Walk the crane forward until the centerline of the front idler tumblers is a few inches past the centerline of the blocking on the mats.
k. Engage the travel locks.
l. Walk the crane back against the travel locks.
m. At this point, the centerline of the front idler tumblers and the centerline of the blocking should coincide.
n. Recheck the radius.
o. Make the lift.

Blocking the tracks at position "a" is not required nor recommended.


Question 3:
Neither type "c" or "d" blocking is required to prevent the crane from rolling backwards during erection.

The travel locks are designed to provide ample resistance to rolling.

There are some engineered lifts, when a crane is equipped with a heavy lift attachment, where sliding of the tracks must be considered. This is when a capacity load is being lifted with a long boom at a low boom angle. For this case, the resisting force generated by the friction between the tracks and the crane mats must be greater than the sliding force generated by the horizontal component of the axial boom force. If this ratio is below 2.5 : 1, chocks similar to "d" must be used to bring it up to a 2.5 : 1 or greater. As stated above, this type of lift would be an engineered lift and is presented here for information only.

    

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