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What is Radial Force?

By Dorothy Distefano
Updated May 17, 2024
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Technically, a radial force is any force that acts in a straight line. In physics, it's often used to describe the influence exerted perpendicular — at a right angle — to the centerline, or axis, of an object traveling in an orbital path. Put simply, a ball swung in an arc at the end of a length of string will experience this force, keeping the string taut. Several other predictable forces work in different directions to keep the ball rotating through the arc, but radial force is responsible for keeping it moving away from the hand holding the string.

Although the exact origins and nature of radial forces are very complicated, they can be seen at work in many everyday processes, such as in power tool bits, rotating car tires, and bearings. In machining, this force is explained as the influence that pushes a cutting tool away from the surface being cut. Correctly calculating radial force specifics is a critical step in the design of tools and other objects with spinning parts.

What Does Radial Force Do?

Radial forces are an important component of the process that keeps any object traveling in a circular orbit. When a known mass (such as a ball) spins around in a circle a set distance (the radius) from the center point at a constant speed, the radial force pushes the mass out, away from the center. The force keeps the mass spinning at the same distance from the center point, maintaining an even orbital path — a circle or an ellipse. Without the action of this force, the path would be erratic and unpredictable.

This is true of all objects in orbit whether they are physically attached to the center or not. A loose ball spun around the inside of a bucket is also forced up against the inner wall of the container by a radial force. Radial forces are, for example, also responsible for keeping a roulette ball up against the edge of the wheel slot while the wheel spins.

Why Are Radial Forces Important?

The accurate calculation of radial forces is a critical design consideration of anything which features orbital motion. Accurately establishing the extent of this variable plays a major role in the overall efficiency, safety, and service life of a piece of equipment, for example. It also allows designers to work out what the practical limitations will be for any given item.

For instance, bearings are widely used in many different types equipment to support, guide, and reduce the friction of motion between fixed and moving machine parts. They are often exposed to radial forces that create internal material stresses that may result in wear and eventual failure if the force and related stresses become excessive. The bearings need to be strong enough to hold up under the pressures that are regularly applied to them. For this reason, there are types of bearings specifically designed and rated for mechanical applications featuring high radial forces.

Another example of this phenomenon at work is the force a cutting tool is subjected to when it removes material from the surface of a work piece. The force acts on the tool to push it away from the piece being cut. The amount of force acting on the cutting device will depend on the features of the tool itself and properties of the material being machined. If the tool is badly designed and experiences an excessive radial force, it could make it very difficult to push the cutting edge into the piece, resulting in sub-standard results or even potentially injuring the operator as the tool is forced away.

Radial Force Variation

A variation of this phenomenon occurs when the momentum of the traveling object changes or evolves during operation. Known as radial force variation, it can be illustrated using the example of an automobile tire. The tire will experience a radial force in a direction perpendicular to the axle of the wheel that, if the tire were perfectly round and the road surface perfectly smooth, would remain the same while the wheel rotates. This is not the case, however, so the radial force exerted on the tire changes every time it rotates. This presents designers with challenges when designing equipment, because it must be able to operate safely under constantly changing environmental conditions.

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Discussion Comments
By anon331221 — On Apr 21, 2013

How do I calculate radial force of bearing for selection?

By nony — On Jun 16, 2011

@MrMoody - Yeah, I think it’s called the centripetal force too. I vaguely remember radial force concepts from helping my kid in school physics. There’s a formula for it; I don’t remember it, but I do get the concept.

If you’ve ever tied a ball to a tether and attached it to a pole, and swung it around in a circle, it’s the same concept.

By MrMoody — On Jun 13, 2011

So if I understand correctly, when I go to the amusement park and jump into one of these g-force rides that whips me around in a circle with a uniform force at a high rate of speed, then what I am experiencing is a radial force?

It’s nice to know the technical name for that thing, although it doesn’t make the ride any less dizzying.

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