Struck capacity is the volume capacity of the bucket after it has been struck at the strike plane. The strike plane passes through the top back edge of the bucket and the cutting edge.
Heaped capacity is the struck capacity plus the volume of excess material heaped on the bucket.
There are two common definitions of heaped capacity:
The heaped capacity of excavator buckets and backhoes are mostly defined with a 1:1 angle of repose (according to standards ISO 7541, ISO 7546, PCSA 37-26, SAE J296/742b).
The heaped capacity of loader buckets are defined with a 1:2 angle of repose (according to standards ISO 7541, ISO 7546, PCSA 37-26, SAE J296/742b and CECE V1).
Fill Factor is the percentage of an available heaped capacity of a bucket that is actually used. For example, a fill factor of 80 percent for a bucket means that 20 percent of the rated volume is not being used to carry material. Buckets often have fill factors over 100 percent.
The fill factors are affected by the bucket design, including the bucket penetration, breakout force, rack back angle, bucket profile and ground engaging tools.
In addition, the bulk material itself has a large influence on the fill factor. A stickier material, such as moist loam, forms easier a good heap than dry, poorly blasted rock.