A short arm is 8 ft. 9 in. in length and has 45,914 lbs. of arm force. A medium arm is 10 ft. 6 in. in length and has 39,930 lbs. of arm force. A long arm is 13 ft. 1 in. in length and has 34,314 lbs. of arm force.
“There is 11,600 lbs. of breakout force difference between the short and the long arm,” says Boyle. “But many contractors are not worried about arm force. They need more reach for finishing the bottom of the trench, pulling the trench box and installing longer sections of pipe.”
Now let’s look at the difference in recommended bucket sizes for material that weighs 2,400 lbs./cu. yd.:
With a short arm, Deere recommends a 2.8-cu.-yd. bucket. With a medium arm, it recommends a 2.6-cu.-yd. bucket. With a long arm, it recommends a 2.4-cu.-yd. bucket.
“In all aspects, it looks like the shorter arm will provide more productivity,” says Boyle. “But in reality, it depends on the application and types of material the contractor is digging. The shorter arm will fill the bucket quicker with hard material; but with softer material, the excavator with the shorter arm will have more moves and more joints in the pipe.”
There are other trade-offs to consider. “A drawback to shorter working equipment is that the working range is reduced, limiting the ability to achieve desired depths or dump heights,” says Yureskes. “The benefit to short boom/arm combinations is that the working forces are closer to the machine, so generally the operator experiences greater stability and also has an expanded range of attachments and bucket sizes that can be utilized if the application requires.”
Sizing the Bucket
Matching a bucket to the application is critical. It is important to realize that the biggest bucket does not necessarily translate into the highest production. Consider bucket fill factors and cycle times, since larger buckets can reduce your cycle speed.
Pellegrini explains that he often captures the application completely before making the bucket recommendation. “When you have a big bucket on the front end, you put a lot of load through the stick and boom. Any kind of twisting or corner loading when a large bucket is placed on a machine can have adverse affects on durability,” he explains. “That is why it is important to match the boom, stick and bucket correctly from the manufacturer’s approved bucket, boom and stick matching guides. And it comes to material density as the deciding factor.”
Material density can vary with weather conditions. “Moisture can play into this, as well, where payload will increase considerably,” says Pellegrini. In some conditions, you might not be able load a large bucket to full capacity as the machine is not matched to the bucket size and the density of material is heavy and hard to get through. In this case, you might be better off choosing a bucket that was sized correctly to achieve a 100% bucket fill factor so payload and cycle times are maximized to the machine model. In other cases, smaller buckets out-produce larger buckets as the machine can cycle faster.
There are also multiple factors involved to properly select a bucket for a given boom or arm. “While all machines will have recommendations for bucket sizing, adding a long boom or arm will generally limit the size of buckets, and a short boom/arm combination will allow for more versatility in bucket selection,” says Yureskes. “Additionally, the application needs to be considered in order to select a bucket that will perform properly, as material density, bucket width and bucket wear packages can affect weight.”
Several other factors can also influence bucket size. “If the customer has installed a coupler or thumb on the machine, the bucket might need to be downsized to keep maximum performance,” says Boyle. “If the customer is loading trucks from a bench with limited reach, a larger bucket may be a consideration. Talk with the dealer or a product specialist to determine the best configuration.”
The Shorter the Better
Short sticks are often the best choice to carry attachments and work tools. “If the stick is too long with a large attachment, you will become stability limited,” says Pellegrini. “We go with our shorter sticks and equip them with auxiliary hydraulics. If you are going to use a thumb, we pull you back to a short stick for stability.”