How to Specify Friction Hinges

Torque Tolerance and Door Moment

What is a friction hinge best used for, and how do you use it to its full potential?

Today in the Sugatsune Hardware Blog, we’re going to be looking at how to get this important detail right in your design plans by accounting for relevant factors such as torque tolerance and door moment. Let’s step into the world of friction hinges.

Quick Overview: What Is a Friction Hinge?

Most hinges deliberately minimize their friction so that the door, lid, etc., on the hinges, can swing as freely as possible along its range of motion. But a friction hinge has the opposite goal: Stop the door or lid from swinging freely (while still keeping it easy for the user to open and close it ).

Why Is Free Movement a Problem?

• Free-swinging doors that open vertically are subject to unintentional slamming and dropping, which can cause damage and injuries. For drops that are only a short distance from the closed position, this risk can be mitigated with a soft-close or dampening hinge, but a more general solution is to prevent the drop from happening at all.
• Free-swinging doors that open to the side are subject to the force of gravity, and due to the door moment, they will tend to swing shut or swing open when it may be preferable for them to stay in place in an intermediate position instead.

The Solution: A Friction Hinge

Friction hinges are a part of our position and movement control hinges product group here at Sugatsune America. We’ve previously written a handy guide on friction hinges, so to learn more about what a friction hinge is, the different types of friction hinges, and how they work, check out The ABCs of Torque Hinges.

Torque Tolerance

Friction hinges inherently have some variability in the exact amount of resistive torque each individual hinge generates. This is an unavoidable byproduct of the technology, and so every model of friction hinge will come with a torque tolerance rating indicating what a friction hinge is capable of actually generating torque-wise.

These tolerances are typically expressed with a plus–minus sign indicating that the true torque value can vary by the stated torque value by that percentage. For example, a torque rating of 25 lbf⋅in ±10% indicates that a given hinge’s true torque will be somewhere between 22.5 lbf⋅in and 27.5 lbf⋅in. On occasion, the tolerances are not symmetrical, and you might see (for example) a tolerance range such as 25 lbf⋅in +20% and -10%. A good manufacturing process will keep the torque tolerance ranges low, usually anywhere from ±25% all the way down to ±5%.

When you are specifying friction hinges in your design plans, you need to account for these tolerances. What a friction hinge is going to need to deliver in terms of resistive torque is something that you can control, but the individual hinges will vary in their performance beyond your control (other than replacing the hinge). You don’t want to randomly get a hinge that’s just a little bit too weak for its application, so it’s important to account for the tolerances when you choose the hinge’s level of resistive torque.

However, you also don’t want a hinge that’s much too strong for its application. If this happens, the door, lid, etc., will be hard to move and potentially even jerky, which can be an inconvenience and sometimes even dangerous.

The Importance of the “Moment” of a Door, Lid, Panel, Etc.

A door’s “moment” is the force of the door rotating around a hinge due to its own weight acting under the force of gravity.

• How to Calculate Moment: Multiply a door’s weight by the distance between its center of gravity and its rotation center. A simpler way is to take the distance of the door, lid or flap from the hinge to the opposite edge, multiply it by its weight and divide by 2.

Every door, lid, panel, etc., has a nonzero door moment. This force is what a friction hinge is going to need to overcome in order to reliably hold the door in place at any angle with free-stop control.

Rule of Thumb for Specifying Friction Hinges

With all of that in mind, here is our rule of thumb:

Make sure your application’s moment is smaller than the low-end torque tolerance value but not hugely smaller.

For example, if you anticipate a moment of 25 lbf⋅in, then a hinge with a 30 lbf⋅in ±10% rating (minimum torque 27 lbf⋅in) will typically do nicely, as would a 35 lbf⋅in ±25% hinge (minimum torque 26.25 lbf⋅in).

It’s very important to have that buffer! After all, what is a friction hinge good for if it isn’t reliable in its free-stop position control?

Get Reliable Free-Stop Control with Friction Hinges from Sugatsune America

Sugatsune America makes hundreds of friction hinges with various torque strengths and features, including basic position types, one-way friction hinges, dual-axis hinges (allowing for more complicated forms of motion), and specialty models such as swivel torque hinges, torque piano hinges. Visit our Friction Hinges page to see a full selection.

Some of our most popular specialty friction hinges include:

• Detent Friction Hinges: Detent hinges stay fully closed rather than springing back slightly.
• Flag and Lift-Off Friction Hinges: The application on these hinges can be easily removed to improve modularity and functionality.
• Swivel Friction Hinges: Swivel hinges allow for additional control, and some models can provide rotation angles up to 360°.

We hope this guide helps you to accurately specify friction hinges in your project. After all, what is a friction hinge good for without someone who needs it?