Reciprocating Grate Drive: Gearbox or Hydraulic?
A reciprocating grate is driven in one of two ways today: gearbox drive or hydraulic drive. Price is rarely the deciding factor; what matters more is whether the fuel stays the same. With a single, steady fuel a gearbox is enough, while a changing fuel, biomass in particular, calls for hydraulic zone control. The two options are compared below by structure, capability and the cases each one suits.
Gearbox drive: simple structure, one speed for the whole grate
A gearbox drive normally uses a gearbox made for reciprocating grates. Where the grate is long and the moving frames are split into groups, an upper and a lower gearbox can drive them separately. On large grates above 40 t/h, a gearbox with a universal joint shaft is also an option: it takes up little space and transmits force smoothly.
One limitation is built in. Stroke and speed are the same across the whole grate and cannot be set zone by zone. With a single fuel of steady moisture and calorific value this is no problem, but it shows as soon as the fuel changes.
Hydraulic drive: one push rod per zone
A hydraulic drive uses one push rod to control one zone, and each zone has its own speed and stroke. The whole system is connected through a PLC to the boiler control system, which gives finer control and makes it easier to interlock with the rest of the boiler.
That is what makes a reciprocating grate workable on biomass. The moisture spread of biomass is very wide. Pelletised or briquetted fuel is very dry and needs a short stay in the preheating zone so that it reaches the main combustion zone quickly, while raw material such as bark carries a lot of water and needs a longer stay in the preheating zone to dry down to a burnable condition. One speed for the whole grate cannot serve both.
| Drive type | Structure | Where it fits | Limitation |
|---|---|---|---|
| Gearbox drive | Dedicated gearbox; upper and lower gearboxes on long grates; gearbox plus universal joint shaft above 40 t/h | Single steady fuel, where one stroke and speed for the whole grate is enough | One stroke and speed for the whole grate, no zone adjustment |
| Hydraulic drive | One push rod per zone, connected through a PLC to the boiler control system | Variable fuels such as biomass, where speed and stroke must be adjusted by zone | The number of push rods sets the size and complexity of the system |
Why fuel differences force zone control
Coal and biomass burn to a different rhythm on the grate. In coal the main combustion zone usually burns volatiles first and fixed carbon afterwards, which takes a comparatively long time, while biomass burns through quickly, releases its calorific value in a burst and finishes sooner.
Add the moisture spread and the same grate can face opposite dwell time requirements in the preheating, main combustion and burnout zones depending on the fuel. Hydraulic zone control exists for exactly this: more time for wet fuel, faster pushing for dry fuel.
How many push rods
The number of push rods sets the size of the system. As a normal arrangement at the partner factory, three rules are usually followed, and the grate drawing and the fuel still decide the final figure:
- Within one level, a furnace width below 2500 mm uses one push rod, and above 2500 mm uses two push rods driving two rows of moving frames;
- A standard inclined reciprocating grate usually has three steps, each about 2000 mm long along the grate surface, driven from top to bottom by three push rods;
- The grate and the pusher feeder can share one hydraulic system, or use two separate systems.
A workable order when choosing is this. With a single steady fuel, take the gearbox first, since it is simple and has few maintenance points. Where the fuel changes often or biomass dominates, take hydraulics and use zone control to burn every fuel through. Above 40 t/h with limited space, a gearbox with a universal joint shaft or upper and lower gearboxes still works. We supply reciprocating grates in all ratings and the matching parts, dispatched from a partner factory, and we can check the drive arrangement together with your grate type and fuel.
