There are a range of boom centres that are employed on modern sprayers. These include rigid, pendulum, trapeze, inverted trapeze, and a combination of pendulum and trapeze.

Rigid or fixed boom centres

For standard boom spraying, fixed or rigid boom centres can have an impact on the boom height, and therefore the nozzle height, particularly when sprayer tyres go over uneven surfaces.

The stability and level of the boom will greatly depend on the chassis and suspension of the sprayer itself, which can make it hard to maintain the optimum nozzle height. Generally, rigid boom centres are better suited to sloping country than self-levelling booms.

Fixed centres may be useful if the operator is considering carrying extra weight on the boom, such as target-selectable units or shielded sprayers.

They are perhaps the best option for attaching some auto height-control systems, as this allows the boom wings to adjust without the centre trying to compensate. Auto boom height can help to maintain nozzle height because there is no compensating movement in the boom centre when the individual boom wings are raised or lowered automatically.

Self-levelling or pivoting boom centres

Self-levelling or pivoting boom centres are generally better suited to wider booms and for machines with less compensation in the suspension. There are several designs available that allow the centre section of the boom to pivot, which allows the boom to remain horizontal (to follow the horizon) as the chassis moves over uneven surfaces.

Tip

If the sprayer has a well set-up pivoting boom centre the boom will want to follow the horizon (due to gravity), so it is very important that there is some way of locking up the boom centre while spraying sloping country and releasing the locking mechanism when spraying reasonably level country.

Pendulum boom centres

Pendulum boom centres have one pivot point in the middle of the boom centre. The pendulum boom centre reduces the impact on the boom wings when tyres are going over uneven surfaces or holes.

Pendulum boom centres are common on some trailing sprayers and some models of self-propelled sprayers.

High pivot points on pendulum boom centres may cause the boom to move sideways when a tyre goes over an uneven surface or down a hole. This sideways shift may create issues in certain situations, such as nozzle alignment over row crops, or maintaining the position of shielded sprayers or target-selectable units, or in controlled-traffic systems. Having a low pivot point may be an advantage, as there will be less influence on the boom and less sideways movement. 

Sensor-based auto boom-height-control systems may not work as intended unless the boom centre can be made more rigid with the addition of boom-roll mechanisms that stabilise the centre section when the individual boom wings are raised and lowered manually or automatically.

Tip

Tilt rams may need to be longer so the boom can be adjusted below horizontal when spraying on uneven surfaces.

Trapeze boom centres

Trapeze boom centres have two level beams with four pivot points – two pivot points on the top of the boom centre and two on the boom mast. The top two pivot points are narrower than the bottom two pivot points. A trapeze boom centre reduces the immediate effect on the boom level when the tyres go over uneven surfaces.

Auto boom height may not work as well unless the boom centre can be made more rigid (for example, with additional boom-roll mechanisms) when the individual boom wings are raised and lowered manually or automatically.

However, when they are fitted with systems that can control the height and level of the centre section, as well as auto height control on the boom (such as the Hardi®–Norac AutoTerrain system), they can maintain boom height in very challenging terrain.

Inverted trapeze boom centres

Inverted trapeze boom centres have two level beams with four pivot points – two on the top of the boom centre and two on the boom mast. The top two pivot points are wider than the bottom two pivot points.

An inverted trapeze boom centre reduces the effect on the boom when the tyres go across uneven surfaces. It also maintains the boom height when going around corners but may struggle coming back to level quick enough.

Auto boom height may not work as well unless the boom centre can be made more rigid when the individual boom wings are raised and lowered manually or automatically (for example, boom-roll mechanisms).

Placing additional weight, such as target-selectable sprayer sensors or shielded sprayers, on this type of boom will require the installation of heavy-duty compensation rams on the centre section to maintain stability.

Combination pendulum/trapeze boom

Combination centres generally have four pivot points (trapeze) – two pivot points on the boom centre and two pivot points on the boom mast. The top two pivot points are narrower than the bottom two pivot points.

For wider booms without auto height-control systems, this may be one of the better designs for maintaining boom stability.

As with other trapeze boom centres this centre design can reduce the impact on the boom and boom level when the tyres travel over uneven surfaces.

Auto boom height may not work as well unless the boom centre can be made more rigid when the individual boom wings are raised and lowered manually or automatically (for example, with the addition of boom-roll mechanisms).