Apron feeder with a Hägglunds direct drive system
Marginal Column

Fact box

Moment of inertia
and shock loads

Moment of inertia is a critical factor in many applications, especially in those involving shock loads. If a sudden stop occurs, the drive’s moment of inertia can place extreme additional torque on the driven machine, straining not only the drive transmission but the shafts, couplings and bearings as well. This stress creates significant wear and tear, with high maintenance costs and reduced productivity as a result.

The fact that a hydraulic direct drive has no gear reducer gives it the greatest protection when it comes to shock loads. (See figure 2.) Its maximum torque can be set at any desired level, thus protecting the drive and the driven machine from shock loads and limiting the stress on machine components, which increases their lifetime and reliability. (See figure 1.)

Moment of inertia
Torque Limitation


The mining and materials handling industry has a keen focus on reliability, performance and productivity. This focus goes beyond machines to the drive systems that power them, such as the hydraulic drives that are increasingly found on apron and belt feeders.


Feeders control the gravity flow of bulk solids, which makes drive selection extremely important for their function. In key ways, low-speed, high-torque hydraulic drives differ from their electro-mechanical and medium-speed hydraulic counterparts.

A well-supported choice

Hydraulic drives, especially of the lowspeed, high-torque type, are the choice for a growing number of apron and belt feeders. While still less common than electro-mechanical drives, these systems can be found in specific installations around the world – and are frequently championed by their operators.

Why the enthusiasm for low-speed hydraulics on apron and belt feeders? The reason is the same as for bucket wheel reclaimers, ship unloaders, car dumper systems, kilns and more. Like all of these, apron and belt feeders operate in harsh environments, where they face both high starting torque and frequent load spikes.

Smarter handling of starting torque

When sizing feeder drives, a major factor is the necessary starting torque. High shear force increases starting torque compared to running torque, often by more than 100% on apron feeders and by at least 50-75% on belt feeders. If coarse ore and larger materials are involved, even more starting torque may be needed. The breakaway torque experienced at start-up can be as much as 200% of the running torque – and sometimes even more. However, hydraulic drives allow precise limiting of the maximum torque, which protects the feeder belts and chains. Direct hydraulic drives are best equipped to take care of high starting torque and torque peaks. The electrical motor always run at synchronous speed regardless of what speed the feeder works with. That means that it is possible to utilize the installed power in optimum way.

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No oversizing of these drives is necessary to cope with the tough feeder starts, as is the case with electro-mechanical drives.

Dealing with low speed and shocks

A further advantage of hydraulic drives is their handling of changes and differences in running speed. At most times feeders are paced for capacity, with small variations due to changes in material density or operating commands. However, major slowdowns can occur when adapting to changes in material flow, or whenever obstructions appear.

Hydraulic drives can run constantly at any speed, from minimum to maximum, without overheating the electric motor. Likewise, they provide built-in protection against shock loads, due to the hydraulic motor’s low moment of inertia.

Easy access to performance

The simplicity of the hydraulic drive chain is also important for maintenance, since feeders are often installed where space is limited. While electro-mechanical drives cover most of one side of the feeder, hydraulic drives leave the drive side largely open, providing easy access for maintenance work. No foundations or physical alignments are required, and there is also less space needed in the axial direction when direct hydraulic drives are used. Given the compactness of the design, it is even feasible to use two hydraulic motors, one on each side of the belt or chain pulley. This makes the load on the feeder structure more symmetrical, yet it still requires only one electric motor and one pump. In addition, it provides a certain redundancy that safeguards production. All this, combined with overall reliability and a long service life, makes hydraulic drives an excellent choice for apron and belt feeders. With their few components, minimal maintenance requirements and outstanding performance, hydraulic drives offer strong assurance of lasting, productive operation.