Through-Hole & Donut Force Sensors: Types, Applications, and How to Choose

 

A through-hole force sensor, also called a donut load cell or load washer, is a ring-shaped force transducer with a central opening for a bolt, rod, or shaft. It measures axial force, most often compression, in assemblies where a solid load cell would block the mechanism. The opening alone does not determine how the sensor must be loaded: the specified loading and support surfaces define the measurement path.

For OEM equipment and research fixtures, start with three questions: which force must be measured, where does that force travel, and how will the complete measurement chain capture it? This guide compares strain gauge and piezoelectric designs, then covers bore and capacity sizing, installation, PLC or DAQ connection, and installed calibration. To see typical formats, browse the FIBOS donut force sensor range.

What Are Through-Hole and Donut Force Sensors?

· Through-Hole, Donut, and Load Washer: How the Terms Relate

Through-hole describes an opening that passes through the sensor. Donut describes its ring-like shape. Load washer usually emphasizes installation around a fastener or within a compressed stack. Suppliers use these names with some overlap, so compare the drawings and installation instructions rather than treating the names as interchangeable specifications. FUTEK's through-hole load cell category illustrates this terminology.

These are force sensors, not fluid pressure sensors. Force is expressed in newtons or kilonewtons. Contact pressure also depends on the effective contact area and its distribution; a force reading does not automatically provide that distribution.

· What Force Does a Ring-Shaped Sensor Actually Measure?

A typical single-axis ring sensor measures an axial force component transferred through its intended sensing structure. In a bolted joint, the sensor may be compressed under the nut while the bolt is in tension. In a flange assembly, the load may enter through an inner annular feature and leave through an outer mounting flange. These arrangements require different tooling.

A center hole does not make a sensor a torque, radial-force, or multi-axis transducer. Nor does it mean a rotating shaft can touch the bore. Confirm the rated measurement direction and provide clearance for the moving parts.

How Do Through-Hole Force Sensors Work?

· Strain Gauge Designs for Static and Dynamic Force Measurement

In a strain gauge design, force slightly deforms an elastic element. Bonded gauges change resistance, and a bridge circuit converts that change into a small electrical signal. With stable excitation and suitable signal conditioning, this approach supports sustained loads as well as changing loads. Dynamic performance still depends on the mounted structure and electronics.

For a bridge rated at 2 mV/V with 5 V excitation, nominal full-scale output is 10 mV. This is an illustrative calculation, not a FIBOS model specification. Such a signal generally needs a bridge input or instrumentation amplifier before a conventional PLC analog input can read it reliably.

· Piezoelectric Force Washers for Dynamic and Quasi-Static Measurement

A piezoelectric force washer generates electric charge when its sensing material is mechanically loaded. A charge amplifier converts this signal into a usable voltage. The approach is useful for fast force changes, while sufficiently slow signal decay and controlled drift can also permit measurements over a defined quasi-static interval. It should not be assumed to hold a stable absolute reading indefinitely.

Kistler explains that quasi-static measurement depends on insulation and the charge amplifier's low-frequency behavior. For a long holding test, compare the acceptable drift and measurement duration with the complete system specification before selecting a piezoelectric sensor.

FIBOS FE011 catalog product image. The charge-output sensor requires compatible charge instrumentation.

Which Through-Hole Sensor Design Fits Your Application?

· Load Washer Designs for Fastener and Clamping Assemblies

A load washer fits within a compressed load path, often around a fastener. Its installation may require specified load introduction washers, surface finishes, or mechanical preload. Confirm the bearing contact area, not just the nominal bolt size. Interface describes its LW load washer as a through-hole product for fastener tension measurement; that application description does not establish compatibility with every joint.

Within the FIBOS range, the FA1339 bolt washer load cell is listed with compression as its loading direction and a 1–2 mV/V rated output, which places it in the bridge-output measurement chain. Check its drawing for bore, bearing area, and required support tooling before matching it to a specific joint.

· Flange-Mounted Designs for Machine Integration

Some ring sensors separate the loading connection from the mounting flange. The central opening accommodates a rod or shaft, while machined annular features and bolt patterns establish how force passes between the machine and the elastic element. FUTEK's LTH500 description, for example, distinguishes inner loading and outer support features. Do not transfer that installation geometry to another model without checking its drawing.

The FIBOS FA128 catalog shows a central shaft passage, an annular loading end, and flange fastening to a fixed seat. It is a useful example when evaluating a machine layout that needs a central opening. Start with the FA128 product page, then confirm the current drawing and mounting arrangement for the selected version. The catalog identifies compression as its loading direction; the hole does not imply tension capability.

FIBOS FA128 catalog product image and compression direction diagram. Confirm the model drawing for the loading and support interfaces.

Mounting geometry and sensing technology are separate choices. A washer-shaped sensor may use strain gauges or piezoelectric material. The FIBOS FE011 catalog, for example, shows a ring-shaped quartz sensor intended to work with charge instrumentation. This is a different electrical measurement chain from FA128's bridge output.

The table below summarizes how the two sensing technologies differ once they are built into a through-hole format. Use it to shortlist a technology, then confirm the selected model's own specifications.

Where Are Through-Hole and Donut Force Sensors Used?

· Bolt Preload and Clamping Force Measurement

A sensor under a nut or bolt head can help measure the compressive reaction associated with bolt preload, provided the intended contact force passes through it. This supports joint-development tests, clamp setting, and comparison of tightening methods. Tightening torque alone is not the same quantity as bolt tension: friction and joint conditions affect their relationship.

State whether the goal is the initial preload, its change during operation, or the external force applied to the assembly. After tightening, a change in washer load need not equal the entire external force because the joint and fastener share load according to their stiffness and contact state.

· Press and Actuator Force Monitoring

A central opening can preserve a tool or actuator passage while a sensor measures axial reaction force. In a press, this can support force-versus-time or force-versus-position monitoring. Decide where the sensor sits relative to the guides, bearings, and tool: friction or another support path between the sensor and the workpiece can make the measured reaction differ from the actual process force.

Loading head and adjustable lower fixture in press equipment.

· Research Test Fixtures with a Central Rod or Shaft

For university and development laboratories, a ring sensor can leave room for a specimen rod, adjustment screw, or coaxial fixture. Define the smallest useful force increment, expected preload, test duration, and required synchronization with displacement or other channels. A small opening or low-profile body does not by itself guarantee useful small-force resolution.

How Do You Choose the Right Through-Hole Force Sensor?

· Match the Bore Diameter, Outer Diameter, and Installed Height

Check the largest passing diameter, including thread crests, tolerances, runout, and assembly access. Then check outer diameter, loading-ring dimensions, mounting holes, cable exit, connector access, and the space needed for washers or adapters. Installed height includes all required load introduction parts, not only the sensor body.

Ask whether the joint remains sufficiently stiff and whether the moving part stays clear throughout its stroke. For a replacement sensor, compare the complete stack and load transfer geometry with the original installation. A matching bore is only one compatibility check.

Selection geometry, schematic only: check the opening, outer envelope, total stack height, and cable or connector clearance together.

· Select Capacity for Preload, Working Force, and Peak Loads

Size the sensor for the largest mechanical load actually reaching it. If a 2 kN preload and a 3 kN process increment act in the same compressive direction, the sensor can carry 5 kN even after the display has been zeroed. This example assumes a direct additive load path; a preloaded joint with parallel elastic paths needs its own load-sharing analysis.

Include acceleration, impact, abnormal cycles, and installation loads. Consult the manufacturer's combined-load and overload limits where relevant. Safe overload is a survival limit under stated conditions, not an extra calibrated working range. For repeated cycling, ask for fatigue information applicable to the actual load spectrum.

Illustrative additive loading: 2 kN preload plus a 3 kN process increment gives 5 kN mechanical load. Zeroing does not remove preload.

· Evaluate Accuracy at the Force Levels You Need to Measure

Read the basis of every error specification: percent of full scale, percent of reading, or another definition. An illustrative 0.2% full-scale error component on a 10 kN sensor corresponds to 20 N. At a 200 N reading, that component alone equals 10% of the reading. It is not the total uncertainty, which also includes other sensor, mounting, and electronics effects.

For small force changes on a large preload, compare noise, drift, hysteresis, creep, and repeatability over the actual cycle. Display digits and ADC bit count do not establish the smallest trustworthy force increment. Ask for evidence at the required working loads and bandwidth.

· Check Bandwidth, Temperature, and Environmental Protection

Natural frequency is not the usable measurement bandwidth. Added mass, mounting stiffness, amplifier filters, and acquisition settings all affect the installed response. Select sampling and anti-alias filtering for the fastest feature that matters, and check whether the PLC scan or communications update can preserve it.

Distinguish operating temperature from the compensated temperature range. Consider thermal gradients from tools or motors, coolant, cable flexing, and connector sealing. An IP rating addresses specified ingress conditions; it does not establish chemical compatibility or eliminate thermal measurement error.

· Through-Hole Force Sensor Selection Checklist

Before requesting a quotation or sample, run through the checks below. Each row pairs the parameter to confirm with the mistake that most often causes a mismatch between the datasheet and the installed result.

How Should You Install a Donut Force Sensor for Reliable Measurements?

· Identify the Loading Surfaces, Support Surfaces, and Load Path

Trace force from the moving component, through the intended loading surface and elastic element, to the machine support. Mark each contact on the assembly drawing. Contact on a protected diaphragm, cable gland, or non-loading surface can produce error or damage. Do not clamp the entire sensor face unless the manufacturer's installation drawing permits it.

Keep shafts and bolts clear of the bore where required, including under deflection. Contact with the bore can introduce side loads or an unintended force path. Route and secure the cable so that machine motion does not pull on the sensor or rub the insulation.

· Control Flatness, Alignment, and Off-Axis Loading

Use the specified hardened washers or machined contact parts, with clean, flat, parallel bearing surfaces. A tilted nut, burr, soft plate, or uneven flange can concentrate contact and bend the sensor. Tighten mounting fasteners in the specified sequence and to the specified values. There is no universal tightening torque for all donut sensors.

Align the force axis with the intended sensing axis and check tool offsets. Resistance to eccentric load is model-specific. A sensor that survives an off-axis load may still produce unacceptable measurement error under that load.

· Account for Preload, Force Shunts, and Assembly Stiffness

Mechanical preload can establish contact and keep a compression-sensitive element engaged. Follow the selected model's preload requirements. Electrical zeroing changes the reference reading; it does not remove preload, restore mechanical capacity, or prove that the assembly has settled.

A force shunt is a parallel elastic path that carries part of a load increment around the sensing element. In a simplified linear assembly with the same incremental displacement, the sensor carries ΔFs = ks/(ks + kb) × ΔF, where ks is sensing-path stiffness and kb is bypass-path stiffness. This model illustrates load sharing; it is not a universal equation for every bolted joint.

Simplified linear parallel-path model with common end displacement. Real joint load sharing must be evaluated for its actual boundary and contact conditions.

HBK's discussion of force shunt measurement explains why the installed sensitivity can differ from the sensor's standalone sensitivity. Establish the assembled relationship with a known reference force. If supports, preload, or contact conditions change, reassess that relationship rather than reusing an old correction factor automatically.

How Do You Connect the Sensor to a PLC or Data Acquisition System?

· Strain Gauge Sensors: Excitation, mV/V Signals, and Amplifiers

For a bridge-output sensor, confirm excitation voltage, bridge resistance, rated output, signal polarity, and the actual pin assignment. Do not infer wiring from cable color alone. A bridge DAQ module can supply excitation and read the differential signal; otherwise use a compatible amplifier to produce the voltage, current, or digital interface required by the controller. FIBOS lists load cell amplifiers for this conditioning step; match their input range and output type to both the sensor and the controller.

Check amplifier input range before tare, output scaling, supply requirements, grounding, and shield termination. Keep low-level signal wiring away from motor drives and switching power cables. Confirm the behavior at overload, sensor disconnection, and power-up so a controller does not mistake a fault signal for a valid force value.

Sensor wiring and a digital indicator at a FIBOS testing workstation.

· Piezoelectric Sensors: Charge Amplifiers and Measurement Time Constants

A charge-output sensor needs a compatible charge amplifier and the specified low-noise cable. Keep connectors clean, dry, and mechanically secure. The sensor's sensing element may require no bridge excitation, but its amplifier still needs power. Do not connect a charge output directly to a standard PLC analog input.

Select charge range and sensitivity settings for the installed sensor, then choose a time constant appropriate to the event duration and acceptable decay. Reset the charge amplifier at a defined cycle condition and document what that reference means. Kistler's charge amplifier technology guide shows how different time-constant settings change the measured waveform.

Two measurement chains: bridge-output sensors need suitable excitation and bridge conditioning; charge-output sensors need a compatible charge amplifier.

How Do You Calibrate and Verify the Installed Measurement System?

· Check Zero, Sensitivity, and Repeatability After Assembly

Record the sensor, electronics, mounting configuration, preload condition, temperature, and filter settings. After warm-up and mechanical settling, check zero at the defined reference state. Apply known forces at several relevant points in the working range, including loading and unloading where practical, and repeat the cycle to expose hysteresis or poor seating.

A shunt calibration resistor on a strain gauge bridge checks an electrical response. It does not apply a mechanical force and cannot verify load transfer, contact quality, or assembly stiffness. Likewise, a stable zero is useful but does not prove sensitivity is correct.

· Verify the Measurement Chain Under Representative Loading Conditions

Where the mounting creates load sharing, calibrate the assembled system against a suitable reference transducer in the intended force path. HBK's CFW calibration procedure distinguishes sensitivity determination after mounting, amplifier setup, and verification. Use the selected sensor's own requirements to plan the procedure.

Then verify the actual measurement task: a holding test for creep or drift, repeated machine cycles for repeatability, and representative transient loading for peaks and timing. A static reference check alone does not establish dynamic response. Define acceptance limits from the process or experiment, and recheck after remounting, overload, or a meaningful change in the force path.

To request a FIBOS selection review, provide an assembly sketch, the passing-part diameter, available space, loading and support surfaces, preload and force cycle, measurement duration, environment, and controller input. If replacing an existing unit, include its full model, drawing, and calibration information. If no standard bore or envelope fits, these details also support discussion of a non-standard customised sensor before committing to a sample.

Frequently Asked Questions

1. Can a Donut Force Sensor Measure Bolt Tension?

Yes, in a suitable assembly it can infer bolt tension from the compressive reaction transmitted through the sensor under the nut or head. The sensor itself may remain compression-only. Check for alternate contact paths and distinguish initial bolt preload from changes caused by an external load; the joint's stiffness determines how those changes are shared.

2. Can a Through-Hole Force Sensor Measure Both Tension and Compression?

Only if its rated loading directions and attachments support both. Some models measure compression only. A preloaded compression sensor may measure a reduction in compression, but that is not permission to pull its sensing element into tension. Check the minimum remaining compression and maximum load throughout the cycle.

3. Does Zeroing Remove the Mechanical Preload on the Sensor?

No. Zeroing changes the displayed or recorded reference. The sensor continues to carry the preload, which still counts toward mechanical capacity. Keep a record of the physical preload and check that the amplifier input has adequate range even when its output has been tared.

4. Can a Donut Force Sensor Replace a Standard Washer Without Changing the Joint?

Do not assume so. Sensor thickness, contact area, stiffness, required load introduction parts, and fastener engagement can change the joint. Evaluate preload retention, available clearance, and the revised load path, then verify the assembly mechanically and metrologically before treating it as equivalent.

5. Can I Replace an Existing Through-Hole Load Cell with One That Has the Same Bore and Capacity?

Those two matches are insufficient. Compare loading surfaces, mounting holes, height, stiffness, direction ratings, overload limits, output, excitation, wiring, environmental limits, and calibration. Treat the new model as a replacement candidate until installation and measurement tests confirm suitability for your application.