I pause. I look at a spinning wheel in my mind. I feel the push fight the rub. Why does motion slow down? Because friction is real. It grips. It resists. It makes a twist harder. That twist fight has a name. We call it frictional torque.
Frictional torque is the turning resistance due to friction. It acts when a shaft turns in a bearing. It acts when a bolt tightens. It acts when a brake pad hugs a disc. It is the reason you must use more effort when parts press tight. A Frictional Torque Calculator helps you find that twist force fast. You input numbers. You get torque. You save time. You avoid guess work.
When to calculate frictional torque
You should calculate it when you design or maintain moving parts. If you pick a motor, you must know how much torque the system needs. If you size a bearing, you must know the friction loss. If you set brake force, you must know how much torque it can hold.
Real life example:
- I am an instructor in a small lab. I guide a learner.
- We have a steel shaft. It turns inside a plain bushing.
- The shaft diameter is 20 mm.
- The normal load on the bushing is 300 N.
- The friction coefficient between shaft and bushing is 0.12.
- The shaft spins, but we want to know the frictional torque that resists it.
- We also want to know the power loss at 600 rpm.
Step-by-step calculation with formula
Step 1: Know the basic formula for frictional torque in a journal bearing:
- T = μ × N × r
- Where:
- T is frictional torque (N·m)
- μ is friction coefficient
- N is normal load (N)
- r is radius of the shaft (m)
Step 2: Convert the diameter to radius in meters.
- Given diameter d = 20 mm
- Radius r = d / 2 = 10 mm = 0.01 m
Step 3: Put the numbers into the torque formula.
- μ = 0.12
- N = 300 N
- r = 0.01 m
- T = 0.12 × 300 × 0.01
- T = 0.12 × 3
- T = 0.36 N·m
Step 4: If you want power loss due to friction at a given speed, use:
- P = T × ω
- ω is angular speed in rad/s
- ω = 2π × RPM / 60
Step 5: Compute angular speed at 600 rpm.
- ω = 2π × 600 / 60
- ω = 2π × 10
- ω = 20π ≈ 62.832 rad/s
Step 6: Compute power loss.
- P = 0.36 × 62.832
- P ≈ 22.6195 W
- So the frictional power loss is about 22.62 W at 600 rpm.
Step 7: If you need torque in different units:
- 0.36 N·m = 360 N·mm
- 0.36 N·m ≈ 3.19 lb·in (since 1 N·m ≈ 8.85 lb·in)
Quick check:
- Higher μ increases T.
- Higher load N increases T.
- Larger radius r increases T.
- The units fit: N × m = N·m. Good.
Why this matters:
- Our motor must handle at least the work torque plus 0.36 N·m extra just to beat friction in the bearing.
- The system also loses about 22.6 W as heat at 600 rpm. We must plan cooling or pick better lubrication.
FAQs
What if I have a disc brake?
Use T = μ × N × R, where R is the effective friction radius of the pad on the disc.
Does speed change frictional torque?
In simple dry or boundary cases, we often treat μ as constant, so T is near constant with speed. In fluid-lubricated bearings, μ can change with speed and oil film, so T can change.
What if I know force at a lever arm?
Torque is T = F × r. If that force is friction force at the contact, it is the same idea.
Final words and a short trick:
Manual trick: T = μ N r. Keep it in your head as “mu times load times radius.” Convert diameter to meters, divide by two, multiply across. For power: multiply torque by 2π RPM / 60.
Why use a calculator: It reduces slips in unit conversion, keeps track of π and rpm math, and lets you test many “what if” cases fast. You avoid under-sizing a motor or over-heating a bearing.