Why Right-Angle Gearboxes Are Larger, And Why They Have to Be
A common question we hear during the specification process is:
“Why is the right-angle gearbox so big compared to a planetary solution?”
or
“Can’t this be done in a smaller gearbox?”
In many cases, the comparison is made against a planetary gearbox rated for the same speed, power and torque. From a purely dimensional perspective, the planetary unit often appears significantly smaller and lighter. However, this comparison overlooks a critical requirement: the need to rotate the drive through 90 degrees.
Understanding why a spiral bevel right-angle gearbox must be larger requires an appreciation of what the gearbox is being asked to do, not just how much torque it transmits, but how it transmits it.
The Fundamental Requirement: Changing the Direction of Power Flow
A planetary gearbox transmits power coaxially, the input and output shafts sit on the same centreline. This allows torque to be increased very efficiently in a compact, cylindrical envelope.
A right-angle gearbox, by contrast, must:
transmit the same power and torque, and
change the direction of rotation by 90 degrees
This change in direction is not a secondary feature, it is the defining function of the gearbox, and it introduces unavoidable geometric and mechanical constraints.
Why a Spiral Bevel Gearbox Needs More Space
Modern right-angle gearboxes typically use spiral bevel gears to achieve high efficiency, smooth operation and long service life. While these gear sets are highly optimised, their geometry fundamentally dictates the size of the gearbox.
Single Load Path at the Angle Change
In a spiral bevel gearbox, all transmitted torque passes through one gear mesh at the point where the axis changes direction. Even with efficiencies of up to 97%, this single load path must be sized to safely carry the full torque.
This requires:
larger gear diameters
wider face widths
and robust shaft and bearing support
By contrast, a planetary gearbox shares the load across multiple planet gears, allowing much higher torque density in a smaller volume, but only when the shafts remain coaxial.
Bevel Gear Geometry Cannot Be “Compressed”
Spiral bevel gears require:
a defined cone distance
precise tooth engagement angles
and adequate spacing to support axial and radial forces
These geometric requirements set a minimum physical size for the gearbox. Unlike planetary stages, they cannot be stacked or nested concentrically to reduce footprint.
Simply put, there is no compact way to turn torque through 90 degrees without allocating space to do so.
Bearing Loads and Structural Requirements
Spiral bevel gears generate significant axial and radial forces as torque is transferred through the angle. These forces must be absorbed by:
larger bearings
increased bearing spacing
and a stiffer housing structure
This is not inefficiency, it is a requirement for reliability, accuracy and service life. Reducing bearing size or housing stiffness to save space would compromise durability and load capacity.
Efficiency Is No Longer the Issue
It is important to clarify that size is not a result of poor efficiency.
Modern spiral bevel right-angle gearboxes operate at efficiencies comparable to planetary gearboxes. The difference in size exists even when efficiency is equal, because the governing factors are:
load path
force direction
and spatial geometry
Efficiency parity does not eliminate the physical requirements of a 90-degree power transfer.
Why Planetary Gearboxes Appear “Better” – Until a Right Angle Is Required
Planetary gearboxes are relatively compact because they:
distribute torque across multiple gears
keep forces balanced
and maintain a straight-line power flow
However, planetary gearboxes cannot change the axis of rotation. The moment a 90-degree output is required, an additional mechanism must be introduced. That mechanism will inevitably add size. A spiral bevel gearbox achieves this function directly, but it must allocate physical space to do so.
Choosing the Right Solution
When a customer specifies a right-angle gearbox, it is because the application requires:
a change in shaft orientation
a defined installation layout
or mechanical integration that cannot be achieved with a coaxial drive
In these cases, the larger size of a spiral bevel gearbox is not a disadvantage, it is the engineering cost of changing direction reliably and efficiently.
Summary
Right-angle spiral bevel gearboxes are larger than planetary gearboxes not because they are inefficient or outdated, but because they perform a fundamentally different task.
Where space allows and a 90-degree change in drive direction is required, a spiral bevel gearbox provides:
high efficiency
robust torque transmission
and long-term reliability
Understanding this distinction helps ensure that gearbox selection is driven by application requirements, not by misleading size comparisons between fundamentally different gearbox architectures.