Run-on torque (also called running torque or prevailing torque) is the resistance you feel when spinning a nut or bolt down the thread before it actually clamps anything together. It’s the torque needed just to move the fastener along the threads, and it produces zero clamping force. Understanding this value matters because it directly affects how tight your joint actually ends up when you’re done tightening.
How Run-On Torque Works
When you thread a nut onto a bolt, the tightening process has two distinct phases. In the first phase, the nut spins freely (or with some friction) down the length of the thread until it contacts the joint surface. During this entire rundown, no clamping force is generated. The parts aren’t being squeezed together yet. Whatever torque your wrench reads during this phase is run-on torque, and it’s essentially “wasted” energy consumed by friction between the threads.
The second phase begins once the nut seats against the joint surface and starts compressing the parts together. This is where actual clamping force (called preload) develops. The total torque you apply at the end is split between overcoming thread friction, overcoming friction under the nut face, and stretching the bolt to create clamp load. Run-on torque is the thread friction portion that exists before any clamping even begins.
Why It Matters for Joint Strength
The standard short-form equation for bolt tightening is T = K × F × D, where T is the applied torque, K is a “nut factor” that accounts for friction variables, F is the desired preload, and D is the bolt’s nominal diameter. The nut factor K rolls all friction effects into a single number, and run-on torque is a major contributor to that value. If run-on torque is higher than expected, less of your applied torque goes toward stretching the bolt, and you end up with lower clamping force than intended.
This is a real problem in practice. If you’re tightening a fastener to a specified torque value but the run-on torque is abnormally high due to damaged threads, rust, or debris, your final clamp load could fall well short of the design target. The torque wrench clicks at the right number, but the joint is undertightened. The reverse is also true: unusually low run-on torque (from heavily lubricated threads, for example) can result in overtightening and bolt failure at the same torque setting.
Common Causes of High Run-On Torque
Several conditions increase run-on torque beyond normal levels:
- Dirty or corroded threads. Rust, oil residue, paint, or debris packed into the thread grooves adds friction that your wrench must overcome before any clamping starts.
- Damaged threads. Burrs, cross-threading, or dented thread profiles create mechanical interference that resists rotation.
- Thread galling. Particularly common with stainless steel fasteners, galling occurs when thread surfaces weld together microscopically during assembly, causing progressively increasing resistance.
- Locking features. Prevailing torque nuts are intentionally designed to have high run-on torque (more on this below).
Cleaning threads and contact surfaces to remove dirt, oil, rust, or debris is considered the most critical step in joint preparation. A clean thread gives you predictable friction, which gives you predictable clamping force.
Prevailing Torque Nuts: Run-On Torque by Design
Some fasteners are specifically engineered to maintain high run-on torque as a locking feature. These are called prevailing torque nuts, and they resist loosening from vibration by keeping constant friction on the threads even when clamp load fluctuates. The most common type is the nylon insert locknut, where a ring of nylon inside the nut deforms around the bolt threads and creates drag.
Metal prevailing torque nuts use a slightly deformed or oval-shaped top section to grip the bolt threads instead. Both types must meet performance requirements laid out in standards like ISO 2320, which specifies minimum torque values that the nut must maintain even after repeated installations. For example, an M10 prevailing torque nut must still hold at least 1.0 N·m of resistance after five removals, down from a first-installation maximum of 10.5 N·m. These values ensure the locking feature hasn’t worn out.
For prevailing torque nuts with non-metallic inserts (like nylon), ISO 2320 requires the maximum first-installation torque to be no more than 50% of the value specified for all-metal designs. This keeps the run-on torque predictable enough that assemblers can still achieve accurate final clamp loads.
How Run-On Torque Is Measured
In production environments, run-on torque is measured during the rundown phase of each tightening cycle, before the fastener seats. Modern electric assembly tools can track torque continuously through the entire tightening process and isolate the run-on portion automatically.
The basic procedure involves first running several test tightenings to establish a baseline. You identify the “seating point,” the angle at which the fastener contacts the joint surface and torque begins climbing steeply. Everything before that point is your run-on torque window. In one documented example, an assembler found the seating point consistently occurred after about 6,300 degrees of rotation (many turns for a long screw), so they set the monitoring window from 0 to 6,000 degrees. The measured run-on torque was consistently near 5 lbf·in, so acceptable limits were set at 4.5 to 5.5 lbf·in. Any fastener falling outside that range during production would be flagged as a failure.
Assembly tools handle run-on torque in two main ways. In “compensate” mode, the tool measures the run-on torque and adds it to the target. If your spec calls for 190 lbf·in of clamping torque and the run-on torque is 60 lbf·in, the tool tightens to 250 lbf·in total. In “monitor” mode, the tool simply records the run-on torque value for quality control without adjusting the final target, which is useful when the spec already accounts for expected friction.
Adjusting Torque Specs for Run-On Torque
When you’re working with prevailing torque nuts or any fastener with significant run-on torque, published torque specifications may or may not include that friction in the target value. This is a common source of confusion. If a spec says “tighten to 80 N·m” and you’re using a nylon locknut with 8 N·m of prevailing torque, you need to know whether the spec means 80 N·m total or 80 N·m of clamping torque on top of the prevailing torque.
Many manufacturers specify that prevailing torque should be added to the listed value. Others build it into the number. If the documentation doesn’t clarify, the safer practice is to measure the prevailing torque of your specific nut and add it to the clamping torque target. Getting this wrong by even a small margin can reduce your actual clamp load significantly, since friction-related losses already consume roughly 85 to 90 percent of the torque you apply to a typical bolted joint. The small fraction that actually stretches the bolt is sensitive to any additional friction stealing energy from the system.