If you’re stepping into the world of mechanical design or engineering, one of the first questions you’ll come across is whether to use spur gears or helical gears. This choice may seem simple, but in reality, it can send ripple effects across your entire project—from efficiency and noise levels to load capacity, complexity, and cost.
But don’t get overwhelmed just yet; throughout this article, we’ll explore what these gear types are, how they’re different, and which kind is right for your project.
But first, let’s build some background.
What Are Gears, Anyway?
Simply put, gears are mechanical parts that distribute motion and torque throughout a machine. They control how fast something turns, how much force it uses, and the direction it moves in. Gears come in a variety of shapes and sizes, but what makes a gear a gear is its teeth.
Each gear has teeth around the edge. The teeth interlock so that if the first gear is turned, the teeth will push against each other. As the gear rotates, the teeth continue to interlock, rotating the second gear as well. The motion from the first gear gets distributed to the second, then the third, and so on and so forth.
Regardless of shape or size, from wristwatches to industrial machinery, gears play an important role. It’s the specifics of their teeth and the shape of their teeth that determine how well the gears perform, and which tasks they’re best suited for.
Spur Gears: Keeping it Simple
Spur gears are the most straightforward or simplest gear design. The teeth have no curves, and they stick out straight from the body of the gear. They mesh together completely, one tooth at a time, making them very efficient.
Spur gears are used for low- or medium-speed applications like conveyor belts, gear pumps, or timing mechanisms in clocks. They’re efficient and inexpensive, but not ideal for quiet or high-speed environments.
Based on a study from Engineering Journal, spur gears are incredibly efficient because they avoid things like sliding friction and axial loads that can complicate other gear types. This simplicity also means that they’re cheaper to produce, so they’re often a very cost-effective option.
Unfortunately, this simplicity does come with downsides. The blunt, straight teeth meshing together often generates a lot of noise and vibration, especially at high speeds. However, a NASA gear-noise study has shown that improvements can be made to a spur gear’s design to help reduce noise. Increasing the contact ratio decreased the spur gear’s noise levels by around two decibels, although these levels are still relatively loud compared to helical gears.
Helical Gears: Quiet Strength
Speaking of helical gears, what makes them different from spur gears in the first place?
Helical gears look similar to spur gears, with the key difference being the shape of the teeth. Helical gear teeth are cut at an angle, whereas spur gear teeth are straight. This means they mesh together gradually, which turns the gear smoother and slower.
This smoother connection creates much less noise and vibration, and can actually distribute the load across multiple teeth. Because they’re quiet and precise, helical gears are often used for automotive transmissions and robotics. Unlike spur gears, which connect one tooth at time, helical gears often have two or more teeth sharing that strain.
There are load distribution studies for helical gears that suggest ideal load-sharing ratios between 33:67 and 45:55, meaning that two teeth work together to share the torque needed to operate. This not only boosts the torque capacity but also makes it less likely that one tooth will be damaged or fail.
However, this seemingly perfect setup comes at a cost: axial thrust. To explain axial thrust, let’s say you have a helical gear with angled teeth. As it spins on its axis—the shaft—the gear teeth turn and mesh with another gear’s teeth. But these teeth don’t just push side to side like spur gears do; because the teeth are slanted, they push against each other at an angle, which creates a sideways force against the shaft. This is called axial thrust.
Think of it like this: If you spin a screw into a piece of wood, the threads (which are like gear teeth) don’t just make the screw spin. They pull the screw forward. A helical gear does something similar; its angled teeth create a force that tries to push or pull the gear along the shaft, not just rotate it.
That backward or forward force (axial thrust) can wear out parts of the machine, or even push gears out of place. So, helical gears need special parts called thrust bearings to absorb and control that force, something that’s not a concern for spur gears.
The Difference Between Spur Gears and Helical Gears
Efficiency and Power Transfer
If your biggest concern is efficiency, spur gears are tempting. Because all the tooth contacts are perpendicular to the axis of rotation, there is very little friction or sliding. This makes them very efficient in simple motion systems, often achieving up to 99% efficiency.
Helical gears do drop by a few percentage points, usually achieving 95 to 98% efficiency. The small loss in energy comes from the effort needed to overcome the axial thrust. Still, well-designed helical gear systems can help to minimize the issue and the loss of efficiency, and in many cases, the benefits of helical gears outweigh the cost.
Load Distribution and Strength
One of the biggest draws of helical gears is how they can share the load across multiple teeth. It really helps to reduce the strain that spur gears can feel with all the pressure put on one tooth. Sharing the load across multiple teeth helps to reduce wear and tear on the gears.
Complexity, Cost, and Maintenance
Because spur gears are simpler, they’re relatively easy to set up or dismantle for maintenance or repair. Their design is simpler, which also makes it more cost-effective; that’s a huge advantage for smaller budgets. And, without axial thrust to worry about, the bearing arrangements are much more simple and can be set up quickly.
Helical gears, on the other hand, require precision. The angled teeth make things more complicated, as they need parts like thrust bearings to compensate for the thrust and careful alignment. These gears cost more up front, both from a manufacturing and a maintenance standpoint, but due to reduced strain, they last quite a while. Reduced noise is also a selling point for some industries.
Choosing between spur and helical gears isn’t necessarily about choosing which is “better” per se; it’s about choosing what’s best for you. Which type fits your needs best? Want low noise, reduced strain, and high torque? Maybe helicals are right for you. Need simplicity and high efficiency, all without breaking the bank? Spur gears might be perfect.
Either way, Lawler Gear is here to help, whether it’s precision machining, custom gearing, or design consultations.
Need help deciding on the right gear for your needs? Reach out to our expert team at Lawler Gear to see the difference decades of experience can make.
