Put a load cell under something heavy and it will tell you, in volts, how heavy. The metal inside gives way by an amount too small to see, gauges riding on it notice, the bridge behind them puts a number on the noticing, and a certificate written against known weights finishes the job by turning that number into pounds.
The question worth asking about any load cell is not "how accurate is it?" but "compared to what?" Accuracy specifications only mean something next to the alternative, and in one very common application the alternative is startlingly poor.
Consider bolted joints. What actually holds a joint together is clamping force — the tension in the bolt pressing the parts against each other. Almost nobody measures it. Instead they measure torque, and torque is only related to clamp through a friction coefficient that varies with thread condition, lubrication, surface finish, plating, and how many times the fastener has been used. Published guidance on bolted joint analysis puts the resulting scatter at roughly ±25% when preload is set with a torque wrench. Tightening by feel is worse still, around ±35%. Turn-of-the-nut methods land near ±15%, and load-indicating washers around ±10%. Only direct measurement gets you into single figures: bolt elongation at ±3–5%, and strain gauge or ultrasonic methods at about ±1%.
Read that list again with a sensor in mind. A device that measures clamping force directly — even one whose own specification looks modest beside a precision test load cell — is operating an order of magnitude better than the torque wrench it replaces. This is why washer-shaped load cells exist as their own product class, and why judging one against a laboratory transducer misses what it is for.
The shape follows the purpose. A washer form goes where a washer goes: over the bolt, inside the joint, in the actual load path. It measures what the fastener is genuinely doing rather than inferring it. And because bolted connections in structures, heavy machinery and aerospace assemblies carry very large forces, this class of sensor reaches capacities most other formats never approach — hundreds of thousands of pounds through a body only a few inches across, which is achievable in heat-treated 17-4 PH stainless and not much else.
Standard mechanism. The interesting part is matching the specification to the decision it supports.
Signal. Excitation goes to the bridge, typically 10 VDC. Compression strains the sensing element, gauge resistances shift, balance is lost, and millivolts appear. Output scales with excitation, so sensitivity is quoted as a ratio. What follows is signal conditioning and then arithmetic: the certificate on file maps that voltage onto a force. Excitation in service must match excitation at calibration or the whole scale is off.
What each accuracy figure is actually telling you. Three numbers, three distinct failure modes. Nonlinearity says how far the response bows away from a straight line between nothing and full load, which bites when your calibration point and your working point are not the same place. Hysteresis says the sensor does not retrace its own path — approach a given load from below and from above and you get two slightly different answers. Nonrepeatability says that even holding everything constant, successive attempts scatter, and this one is stubborn: no amount of calibration removes it. Which one hurts you is decided entirely by what you do with the reading.
Why monitoring and metrology have different requirements. If you are certifying a force value that someone else will rely on, every one of those figures matters and you should buy the tightest you can. If you are watching whether a bolt has lost clamp since it was installed, the question is different: you need to detect a meaningful change reliably, which puts the weight on nonrepeatability and on zero stability rather than on absolute linearity across a range you never visit. A sensor optimised for the second job can look unimpressive on paper while being exactly right in practice.
The washer form has its own installation rules. Load must arrive evenly around the ring, which means the surfaces bearing on each face need to be flat, hard, and sized to the sensor rather than to the bolt. The bore is clearance only — a fastener rubbing the inside wall adds friction and side load the element cannot separate from clamp. And because a washer sits inside the joint, it becomes part of the stack: its thickness has to be accommodated in the available grip length, and it compresses fractionally under load like any other member.
Zeroing decides what you are measuring. Zero before the joint is tightened and you record the clamp developed as you torque. Zero afterward and you have subtracted that clamp, leaving only subsequent change. For long-term monitoring the second is often what you want; for verifying assembly, the first. Choose deliberately, and stay consistent, because changing convention partway makes historical readings incomparable.
Selecting a washer-type sensor starts with an honest answer about what the number is for. Our application engineers can place you quickly once that is settled.
Decide whether you are measuring or monitoring. Certifying a force for a report, a customer, or an auditor calls for the tightest specification you can justify. Confirming that a joint was assembled correctly, or that it still holds clamp two years later, is a different task — you need to reliably detect change rather than certify an absolute value, and a sensor built for that job will cost less, fit better, and reach capacities a precision transducer never will. Buying the wrong one in either direction is expensive.
Compare against the method you are replacing, not against a catalogue. If today's process is a torque wrench, you are living with roughly ±25% uncertainty on the thing you actually care about. Almost any direct measurement of clamp is a substantial improvement on that. Judging a fastener sensor by whether it matches a laboratory load cell asks the wrong question.
Work out the physical fit before anything else. What passes through the middle sets your minimum bore, the space around the fastener caps your outside diameter, and the available grip length has to accommodate the sensor's thickness plus its bearing surfaces while leaving proper thread engagement. That last one is the constraint people discover last and should check first.
Size capacity against what the joint could see. Not the design clamp — the force a technician with a long bar can generate, or a powered tool set slightly high. Overload during assembly is the characteristic failure mode in fastener measurement, and it happens in seconds.
Specify the bearing surfaces. They want hardness, flatness, and enough diameter to sit across the whole ring on both faces. Hardware pulled from the fastener bin usually fails on one count or the other — either too small to reach the outer part of the annulus, or soft enough that it dents progressively and moves the contact patch around while the load is building.
Then environment and readout. Sealed gauge construction handles dust, debris and moderate moisture; genuinely wet or corrosive service is a different specification worth discussing. Compression calibration covers pushing only. And for monitoring installations the readout matters more than usual: an amplifier signal conditioner module or a digital display with setpoint alarms turns a reading nobody watches into a notification somebody receives, which is generally the whole point. Cal-Teds plug and play keeps calibration attached to each unit where several are installed.
Give us the bolt or rod diameter, the space around it, the clamp force involved, and what decision the reading supports, and we will point you to the right sensor. Standard items are held in stock, and educational pricing is available.
LWO Series Load Cell Applications.
The Transducer Techniques LWO Series washer-shaped, strain gauge-based load cells are specialized devices with applications in fastener testing and through-hole load measurement scenarios.
- Fastener Testing: The LWO Series load cells are commonly used in fastener testing applications, where they measure the compressive forces experienced by bolts, screws, and other fasteners during tightening, torque testing, and quality control.
- Structural Testing: These load cells are applied in structural testing scenarios, including load-bearing bolted connections in buildings, bridges, and other infrastructure.
- Construction and Civil Engineering: In the construction and civil engineering industries, LWO Series load cells are used to assess the integrity of anchor bolts, foundation bolts, and other critical connections.
- Heavy Machinery and Equipment: The load cells find applications in the testing of heavy machinery and equipment that relies on bolted connections for structural integrity.
- Aerospace and Aircraft Assembly: The aerospace industry uses LWO Series load cells to measure the compressive forces applied to aircraft fasteners, ensuring that connections in critical areas are secure and compliant with safety standards.
- Automotive and Manufacturing: In manufacturing and automotive assembly lines, LWO Series load cells are employed to monitor and control the torque and load applied during the assembly of components.
- Research and Development: Engineers and researchers use these load cells during product development and testing to evaluate the performance of fasteners and through-hole connections in various applications.
- Quality Control: Quality control processes in various industries, including aerospace, automotive, and manufacturing, benefit from LWO Series load cells to verify that fasteners meet specific load and torque specifications.
- Load Monitoring: The load cells can be used for ongoing load monitoring of bolted connections in structures, machinery, and equipment to ensure they remain within safe operational limits.
- Educational and Training: Educational institutions incorporate LWO Series load cells into engineering and materials science laboratories to teach students about bolted connection mechanics and conduct experiments related to fastener testing.
Their reliability and precision make them valuable tools for force measurement in demanding applications.
Frequently Asked Questions
What is the LWO Series, and what is it built for?
The LWO Series are washer-shaped, strain gauge based load cells built specifically for fastener testing and through-hole load applications. Rather than being a general-purpose test transducer adapted to bolted joints, the washer form exists because that is where the measurement needs to happen — over the bolt, inside the joint, in the actual clamping load path. The series spans 2,400 lb up to 300,000 lb, which is the range bolted connections in structures, heavy machinery and aerospace assemblies actually operate in.
Why are the LWO's accuracy specifications looser than your other load cells?
Because it is answering a different question, and the right comparison is not another load cell. Nonlinearity and hysteresis are each 2.0% of rated output, nonrepeatability 1.0%, and zero balance 1.0%. Set against a precision test transducer that looks modest. Set against how bolt clamping force is usually determined, it is a transformation: published bolted-joint guidance puts torque wrench preload accuracy at roughly ±25%, and tightening by feel at around ±35%. A sensor measuring clamp directly at these specifications is an order of magnitude better than the method it replaces. That is the standard it should be judged against.
What capacities does the LWO Series reach?
From 2,400 lb through 300,000 lb — 150 tons at the top end, which makes this one of the highest-capacity series we build. Bolted connections in bridges, foundations, presses and heavy structures generate forces on that scale, and the washer format is one of very few ways to measure them without redesigning the joint around a sensor.
What sizes are available, and how do bore and outside diameter relate to capacity?
Outside diameters run from 0.85 inches up to 3.75 inches, with bores from 0.192 inches up to 2.041 inches, and each model pairs a specific bore, outside diameter and capacity as a set. That's a different selection process from a fixed-body series with interchangeable bore options — here you identify the model whose combination of hole, envelope and load rating suits your joint, rather than choosing dimensions independently. Send us your bolt size, available space and clamp force and we'll identify which model fits all three.
What is the LWO's deflection, and why does it matter in a bolted joint?
0.001 inches at rated output, which is the lowest deflection figure in our range. That matters more in a fastener application than almost anywhere else, because the sensor becomes a member of the bolted stack. A component that compresses significantly under clamp would alter the joint's stiffness and change how it behaves — one thousandth of an inch is small enough that adding an LWO to a joint doesn't meaningfully change what you're trying to measure.
Are the LWO's strain gauges sealed?
Yes, the bonded foil gauges are sealed, which suits a sensor likely to be installed in a structure or machine and left in place for long-term load monitoring. To be precise about what that means: this is protection against dust, debris and moderate moisture rather than hermetic sealing, so outdoor structural exposure, wash-down or corrosive conditions are worth discussing with us specifically rather than assuming coverage.
Is the LWO compression only?
Yes, standard calibration is compression, which matches how the sensor works in service — captured under a nut or bolt head and squeezed as the joint is tightened. That's the natural direction for a load washer, since clamping force always compresses whatever sits in the joint. If your application involves anything other than compression, raise it with us before ordering.
How does temperature affect an LWO reading?
Compensation runs 60° to 160°F with a safe range of −65° to 200°F; inside the compensated band, output drifts 0.005% of load per °F and zero drifts 0.01% of rated output per °F. This is worth calculating in advance on structural monitoring in particular, where seasonal temperature swings are large and slow — without that arithmetic it's easy to read a thermal cycle as a joint gradually losing clamp.
What is the LWO made from?
Heat-treated 17-4 PH stainless steel. At 300,000 lb through a body under four inches across, the material is doing extraordinary work — 17-4 PH is a precipitation-hardening stainless whose heat treatment forms fine strengthening particles throughout the metal, raising strength several times over standard grades without the distortion that would ruin a precision component. Very little else would carry that load in that envelope.
Can I get the LWO with the plug-and-play TEDS option?
Yes, OPT-TEDS is available and holds the unit's calibration data at the connector so a compatible instrument applies it automatically. Because LWO models pair specific bores, diameters and capacities rather than sharing one calibration across a series, that's genuinely useful when several different units are installed across a structure or machine — it removes the possibility of a sensor being read against the wrong figures.
Questions From The Field
Will an LWO actually improve on our torque wrench procedure?
Almost certainly, and by more than the specifications suggest. Torque is an indirect route to clamping force, connected to it only through a friction coefficient that shifts with thread condition, lubrication, plating and reuse — which is why published guidance puts torque wrench preload accuracy near ±25%. An LWO measures clamp itself rather than inferring it. Even accepting its own specification figures, you move from a quarter-scale uncertainty to a few percent on the quantity that actually holds your joint together.
Should I use an LWO or one of your through-hole donut cells?
Ask what the number is for. If you're certifying a force value — a report, a customer requirement, an audit trail — the tighter specifications of the through-hole series are worth the money. If you're verifying that a joint was assembled correctly or watching whether it still holds clamp, the LWO is purpose-built for that and reaches capacities the donut series doesn't approach. There's also a practical dimension: at 300,000 lb, the LWO may simply be the only option in the range.
How do I tell whether a change in reading is real or just sensor drift?
Work through it in order. First calculate what temperature alone would explain from the published coefficients and compare that against your measured change — on outdoor structures this accounts for a great deal. Then consider timing: joints relax fastest in the weeks after assembly as surfaces bed in, so a decline that's decelerating suggests normal settling while one that's steady or accelerating suggests something else. Finally, nonrepeatability of 1.0% of rated output sets your practical detection threshold: a change smaller than that isn't reliably distinguishable from scatter, so build your alarm limits with that in mind.
What clamp change should I set an alarm at?
Above the sensor's own scatter, and below whatever your joint design considers a problem. With nonrepeatability at 1.0% of rated output, an alarm threshold well inside that figure will trigger on noise rather than on events. Work from the other direction too: establish what clamp loss your engineering considers significant, and confirm it's comfortably larger than the sensor's detection floor. If those two numbers overlap uncomfortably, that's a sign you need a tighter sensor or a lower-capacity one where the same percentage represents fewer pounds.
My joint doesn't have enough grip length for the sensor. What now?
This is the most common obstacle in retrofitting fastener measurement, and it usually has a solution. Options include a longer fastener of the same grade, a redesigned spacer arrangement that gives back the height the sensor consumes, or a different LWO model with a slimmer profile at the capacity you need. What you should not do is proceed with reduced thread engagement to make room — a joint that fails because the bolt was short defeats the purpose of measuring it. Send us the joint details and we'll work through the options.
The bolt is contacting the inside of the washer bore. Does that matter?
Yes. The bore is clearance and nothing else — a fastener bearing against the wall introduces friction and lateral force that arrives at the gauges mixed in with the clamping load you want, with no way to separate them afterward. On a bolted joint this is easy to miss because the bolt is captive and appears to be sitting correctly. Check for genuine clearance all round, and if a model's bore is marginal for your fastener, move to one with a larger hole rather than accepting the contact.
Can I leave LWOs installed on a structure for years?
Long-term load monitoring of bolted connections is one of the intended uses, and the sealed gauge construction supports it. Two provisions make it work. Confirm the environment sits within that protection level — outdoor structural exposure over years is more demanding than a factory floor and worth discussing with us. And record the reading at commissioning as your reference, because on a monitoring installation the meaningful data is the difference from that starting point rather than any single value.
How do I verify an LWO that's been installed a long time?
The practical in-place check is comparing the unloaded zero against what was recorded when the sensor was new, which is why capturing that figure at commissioning is worth the moment it takes. Beyond that, set a calibration interval appropriate to how much the reading matters — more frequent where it supports a safety function or a structural assessment. On a bolted joint you also have a useful cross-check available: if the joint can safely be released and re-torqued to a known value, seeing whether the sensor returns to its expected reading separates instrument drift from genuine clamp loss.