"To be productive, you will want to get the largest bucket you can for the material and working conditions," says Horton. "If you have light, soft material, you can get away with a larger bucket. If you get to very hard soil or even rock or compressed conglomerate material, you need a shorter, smaller bucket to keep your breakout force high enough."
"What can be a selling point on a machine that might have a higher breakout force based on horsepower can be negated, or made up, [based on] the bucket or bucket/coupler combination," adds Gabriel. "That would take away, or add to, a competitive advantage."
Lift capacity of a given machine also affects the size of the bucket, says Kirk Yoresen, marketing communications manager, ESCO. Be aware that stick configurations can affect load, i.e., excavators with extended booms can carry less weight.
"You want to make sure you're not asking the bucket to do too much for the available machine power," he adds. "Performance goes down and safety can become a concern.
"If bucket capacity goes beyond the lift capacity, stability issues can come into play," Yoresen continues. "If an operator is working on a high wall loading heavy material, good stability becomes much more important for safety."
A bucket that's too large can also decrease breakout force, since the tip radius (distance from the stick pivot to the tip of the teeth) is in direct inverse proportion to the amount of force you get when you dig.
"The shorter the tip radius, the higher the force at the tooth tips," explains Horton. "Also, the fewer the number of teeth you have engaging the material, the more force you have on the tooth tips. With our multi-ripper bucket, we have one tooth hitting at a time. Plus, it's short, so when you roll the bucket, you can dig through frozen ground and rock."
Sizing the bucket also includes taking the width into account. Width can be of particular concern if you are digging trenches for utility lines. "You don't want to dig more than you have to," Horton states. "If you dig too much, you're wasting time and money."
Wide buckets can be used in soil that is easy to dig, but you will likely want a narrower bucket for harder soil conditions. That's because the stick and curl forces of a wider bucket are spread over a larger area, so there is less penetration per inch of bucket cutting edge.
When assessing bucket width, also be sure to check the width of the connection point. "You may have a particular size machine and not realize that the connection at the top of the bucket is wider than the bucket width you want to use," says Tony Freidhoff, engineer, Kenco.
Effect of couplers
Couplers give you the ability to change buckets based on the application at hand. "A coupler makes it faster and easier to change buckets," says Horton. "Once you make it faster and easier, you can pick the right bucket for the job."
On the other hand, it's important to take notice of which coupler you're using. Depending on the type, it can change the geometry of the machine and reduce its breakout force.
A dedicated coupler, which is integral to the machine, has minimal affect since it maintains the highest possible digging power. "In this scenario, virtually no other aspects come into bucket selection because the tip radius of the bucket and the machine remains the same," says Cveykus. "The breakout force, bucket rotation, etc., of the machine all remain virtually the same as the OEM specifications."
Adding a hydraulic pin grabber coupler, on the other hand, could move the bucket farther away from the machine, adding to the tip radius. "That could reduce the breakout force, depending on the bucket and machine configuration," says Gabriel.
Tip radius measurement affects the bucket curling force. A bucket with a short tip radius provides more curling force than one with a longer tip radius, facilitating easier loading of material.
Your bucket manufacturer can help you calculate the affect a coupler might have on the breakout force of your machine. "You certainly want to account for it so you don't tip the machine," says Yoresen. "You need to be aware that there will be a reduction in breakout force because you're changing the geometry of the machine relative to the bucket. When you change the geometry - the relationship of the bucket arm to the tip of the bucket - it affects the available machine power."