⚡ How to Measure Brush Spring Thrust Accurately: An IEC TR 61015 Engineering Guide

Published: 2026-05-16 | Standard: IEC TR 61015:1990 | Domain: Rotating Machinery / Brush-Holders

In every rotating electrical machine that uses carbon brushes — whether a DC motor, a DC generator, or a wound-rotor induction motor — brush spring thrust is the single most influential parameter governing commutation quality and brush service life. Too little pressure, and the brush loses contact with the commutator, producing destructive arcing and surface burning. Too much pressure, and both the brush and the commutator wear at an accelerated rate while operating temperature climbs. IEC TR 61015:1990 is the international technical report that standardizes exactly how this deceptively simple measurement should be performed in the laboratory.

1. Why Brush Spring Force Is the Lifeblood of Machine Reliability

Carbon brushes are simultaneously the most overlooked and the most critical wear components in a rotating machine. A brush operates under the delicate balance of three forces: the mechanical thrust from the spring system, the frictional drag from the rotating commutator or slip ring, and the electrodynamic forces created by current transfer across the brush-to-copper interface.

1.1 The cascading failure of insufficient thrust

When spring thrust falls below the design value, the electrical contact between the brush and the commutator becomes unstable. At the microscopic level, the number of conducting a-spots drops sharply, forcing the remaining contact points to carry disproportionately high local current densities:

• Arc erosion — Dark burning streaks appear on the commutator surface (often called “fingerprint” or “chicken-scratch” patterns). In severe cases, continuous erosion grooves form around the commutator circumference.
• Elevated sparking grade — Commutation sparking rapidly escalates from the normal grade 1¼ to grade 2 or even grade 3, potentially triggering fire-detection monitoring systems and causing unplanned shutdowns.
• Patina destruction — The brown cuprous oxide (Cu₂O) protective film on the commutator surface is stripped away by arcing, exposing bare copper. This leads to erratic friction coefficients and can initiate a “brush bounce” vicious cycle.

1.2 The hidden cost of excessive thrust

A persistent intuition among maintenance personnel is that “a little more pressure is better than a little less.” This is a dangerous misconception:

• Accelerated mechanical wear — Carbon brush wear rate is approximately proportional to spring pressure. Increasing specific pressure from 18 kPa to 30 kPa can reduce brush life by 30% to 50%.
• Commutator temperature rise — Frictional power (P = μ × F × v) increases linearly with thrust. For a medium-sized industrial DC motor, an extra 5 N of thrust can raise commutator temperature by 8–15°C.
• Excessive carbon dust — Over-compressed brushes “crumble” under excessive load, producing large quantities of conductive carbon dust that contaminate winding overhangs, degrade insulation resistance, and can even cause electrical tracking failures.

1.3 Recommended specific pressure ranges

IEC TR 61015 defines the specific pressure calculation as:

where F is the measured static force (N) and A is the true cross-sectional area of the brush box (mm²). Critically, the actual contact area of the brush on the commutator or slip ring is not used in this calculation. The brush box area is a fixed design value, whereas the contact area changes continuously over the brush’s life (bedding-in, stable wear, end-of-life). Using the box area ensures consistent, comparable measurements across brush-holders and over time.

2. The IEC TR 61015 Test Method — Key Engineering Details

IEC TR 61015 describes a carefully designed laboratory measurement procedure. Its central idea is elegantly simple: use controlled vibration to eliminate static friction (stiction) interference, approach the measurement point from two opposing directions, and average the readings to obtain a repeatable static thrust value.

2.1 The five essential test rig components

The standard specifies that the test rig consists of five fundamental elements:

1. Measuring device (1) — Must be a balance or load cell with an accuracy not worse than ±1%. The part to which the load is applied (1.1) must have a displacement no greater than 0.2 mm under load. This explicitly rules out ordinary spring balances: their internal displacement typically measures several millimeters, which would significantly alter the compression state of the spring being measured.
2. Support (2) — Positions the brush-holder so that the axis of the brush box is precisely aligned with the direction of travel of the measuring device. Any angular misalignment introduces cosine error into the measurement.
3. Slide (2.1) — Provides displacement parallel to the brush axis, enabling the brush-holder or brush to travel between chosen limiting points (2.2 and 2.3). This simulates the full stroke range from a new brush to a fully worn brush.
4. Vibration devices (3) — This is where the standard demonstrates genuine engineering insight. Two vibrators operate at fixed frequencies selected from the range 50 Hz to 120 Hz, with a variable amplitude of at least 0.05 mm minimum. The purpose of vibration is to overcome the static friction between the test brush and the brush-box walls, allowing the spring thrust to “relax” to its true value. Either one or both vibrators may be used, determined experimentally for each measurement setup to achieve optimum stabilization.
5. Test brush (4) — Made of steel, not carbon. This is a deliberate choice to ensure uniform, reproducible results (carbon brush materials exhibit inherent variability in density, surface hardness, and friction characteristics across production batches). The head shape conforms to the brush-holder manufacturer’s specification. The base includes a point of support on the axis of travel. Critical dimensions t and a use the nominal values of a real brush but are machined to e6 tolerance (precision fit), while dimension r is not less than the maximum r-dimension of a real brush accepted by that brush-holder.

2.2 Optimum vibration stabilization — the standard’s core innovation

The “optimum stabilization” procedure described in Clause 5.3 is the most innovative aspect of this standard. The practical steps are:

1. Slide the test brush toward the chosen measuring point in one direction, immobilize it, apply vibration, and record reading F₁.
2. Slide the test brush toward the same measuring point from the opposite direction, immobilize, apply vibration, and record reading F₂.
3. Adjust the vibration amplitude and/or frequency (or switch between the two vibrators) and repeat until the difference between F₁ and F₂ is minimized.
4. When F₁ ≈ F₂, optimum stabilization is achieved. The static thrust F = (F₁ + F₂) / 2.

The physics behind this procedure: when a brush slides inside its box, the direction-dependent friction introduces a bias into the measured spring force, analogous to the mechanical “backlash” phenomenon in instrument gears. By approaching the measurement point from both directions and using vibration to release the stored friction, the frictional bias is effectively averaged out, revealing the true static spring thrust.

Figure 3 of the standard elegantly illustrates this concept with a force-versus-position diagram: the upper curve (F₂) and lower curve (F₁) form a hysteresis envelope. The mean line between them, F = (F₁ + F₂) / 2, represents the true static force characteristic as a function of brush position from new to fully worn.

2.3 Streamlined batch acceptance testing

The note in Clause 5.5 provides a practical industrial concession: for acceptance testing of a production batch of brush-holders, the full multi-position measurement procedure is first performed on a representative sample. Once the thrust characteristic is established for that sample, the measurement for the remainder of the batch may be limited to a single brush position. This significantly improves inspection throughput in high-volume manufacturing while preserving statistical quality control effectiveness.

3. Common Brush Maintenance Pitfalls and Engineering Best Practices

3.1 Pitfall: “New brushes will bed in naturally”

Freshly installed brushes must undergo a deliberate seating (bedding-in) procedure to grind the brush face to match the commutator’s curvature. Without proper seating — using a seating stone or glass-cloth abrasive strip pulled between the brush and commutator — the initial contact area may be only 10% to 20% of the nominal value. This concentrates the full current through a tiny contact zone, producing severe sparking and localized commutator overheating. After seating, the static thrust should be re-measured because the new brush height usually differs from the test brush height, changing the spring compression.

3.2 Pitfall: “Set it and forget it”

Both helical coil springs and constant-force spring mechanisms experience stress relaxation over time. In machines operating at elevated temperatures (commutator-zone temperatures can reach 120–150°C), the elastic modulus of spring materials gradually degrades. For a continuously operating industrial DC motor, spring thrust can decay by 10% to 15% within two years. This is why the IEC TR 61015 measurement methodology should also be periodically applied to in-service brush-holders as part of a condition-based maintenance program.

3.3 Pitfall: “All brush positions should get the same pressure — close enough is fine”

In multi-brush machines, thrust uniformity across all brush positions is critical. Industry practice typically calls for ±10% maximum deviation within the same polarity group. Non-uniform thrust distribution causes some brushes to carry a disproportionate share of the total current (lower contact resistance from higher thrust attracts more current), accelerating those brushes’ wear and creating a “weakest link” failure pattern.

3.4 Engineering best practice checklist

• ✅ Use a calibrated load cell (±1% accuracy), never a spring balance, for thrust measurement.
• ✅ Measure at three brush positions: new, half-worn, and fully worn limit.
• ✅ Always apply vibration stabilization (50–120 Hz, ≥0.05 mm amplitude) to eliminate stiction.
• ✅ Record and trend thrust values; build a thrust-versus-life profile for each machine.
• ✅ Inspect spring condition during every brush change — replace springs showing rust, deformation, or discoloration (overheating evidence).
• ✅ Use brush-box cross-sectional area (mm²) for specific pressure calculation, never the commutator contact area.

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