An enclosure drop test failure needs a controlled stopping distance, not simply a stronger outer box. When fragile internals cannot change, isolate their supporting frame from the enclosure in every required direction. Then prove that the assembly stays within its shock limits without striking a wall or hard stop.
For a 4 in [102 mm] drop, start with measured enclosure and payload acceleration. Combine those records with the suspended mass, allowable shock response and usable travel. A mount that fits physically may still be too stiff, too soft or constrained by its attachments.
What the damage pattern tells you
Inspect the failed assembly before changing mounts. Disconnect power, secure the enclosure and document damage without disturbing witness marks.
- Buckled flexures with an undamaged shell: The enclosure may have transmitted a damaging pulse without absorbing much impact energy. Shell survival does not establish payload protection.
- Damage concentrated beside one support: Unequal load sharing, local frame bending or rotational motion may be overloading that location.
- Scrapes, dents or compressed bumpers inside: The isolated assembly may have exhausted its travel. A hard-stop impact creates a second shock event.
- Failure in only one orientation: Mount stiffness, preload, gravity sag or available clearance may differ between directions.
- Cracked sensitive elements without visible movement marks: Local resonance or a short acceleration pulse may be responsible. Peak acceleration alone cannot distinguish them.
- Failures appearing after repeated drops: Check for mount migration, fastener movement, permanent deformation and cumulative internal damage.
Record which face, edge or corner contacted the impact surface. Also record the assembly orientation and the sequence of previous drops.
Loose terminals and displaced relays require a different investigation. The article on enclosure transport testing for loose wiring and components addresses those assembly-retention problems. Here, the central question is how to limit shock reaching a fixed, fragile payload.
Check the likely causes in order
Start with the shortest mechanical path between the impact surface and the damaged assembly. Follow each load path through the shell, supports and payload frame.
Direct transmission through rigid supports
A hard-mounted frame follows the enclosure's rapid stop. Stronger fasteners may retain it more securely without reducing the transmitted acceleration.
Compare synchronized acceleration records at the enclosure and payload frame. Similar pulses suggest little isolation, although local structural response can modify either signal.
The corrective action is a compliant support system around the complete fragile assembly. Confirm first that its frame can carry the resulting distributed support loads.
Travel exhausted before the payload stops
A soft mount can reduce the initial pulse but allow excessive relative movement. Contact with a wall, bolt or stop then defeats the isolation.
Check clearance in both directions along every axis. Look for contact marks and measure motion during controlled tests; visual inspection alone cannot exclude brief contact.
Increase usable travel, revise the mount characteristics or introduce an engineered progressive stop. Do not assume adding a thin bumper provides sufficient stopping distance.
Isolation bypassed by attachments
A stiff cable bundle, grounding braid, thermal strap or rigid pipe can bridge the isolation system. Its effect may appear only near maximum displacement.
Inspect each attachment through the full motion envelope with power disconnected. Provide suitable flexible routing, bend radii and strain relief without compromising electrical safety.
Directional stiffness or structural resonance
Many mounts behave differently in compression, tension and shear. A satisfactory vertical result does not establish protection during a side impact.
Compare directional mount data with the actual installation. If damage tracks pulse duration rather than peak acceleration, investigate the payload's structural modes and shock response spectrum.
Calculate the travel before choosing mounts
A 4 in [102 mm] free fall produces an impact speed of approximately 4.63 ft/s [1.41 m/s]. This follows from v = sqrt(2gh), neglecting air resistance.
The corresponding kinetic energy is about 1.00 J per kilogram of falling mass. That energy must go somewhere during impact and subsequent internal motion.
For a first estimate, assume the enclosure stops abruptly while the isolated payload initially continues moving. An ideal constant-deceleration stop gives:
a = v² / (2s)
Here, s is the payload's stopping distance relative to the enclosure. Neglecting gravity during that stopping stroke, average deceleration expressed in g becomes approximately h / s.
| Usable stopping distance | Idealized average deceleration after a 4 in drop |
|---|---|
| 0.10 in [2.5 mm] | 40 g |
| 0.20 in [5.1 mm] | 20 g |
| 0.40 in [10.2 mm] | 10 g |
These are planning estimates, not predicted acceleration peaks or acceptance limits. Real spring forces vary throughout the stroke, and actual peak acceleration can be substantially higher. Gravity also affects vertical loading, while enclosure rebound changes the relative motion.
Use the table to reject impossible packaging assumptions. If the payload needs a gentle stop, a very small gap cannot provide it without another energy-absorbing mechanism.
Separate physical gap from usable travel
The wall-to-frame gap is not entirely available for stopping. Subtract static displacement, assembly tolerances, mount creep allowance, rotational sweep and a verified no-contact reserve.
For a simple linear vertical support, static sag is mg / k. Its natural frequency is fn = sqrt(k / m) / (2π).
An ideal support with a 10 Hz vertical natural frequency has approximately 0.098 in [2.5 mm] static sag under gravity. That sag consumes clearance before any drop occurs.
Use the complete suspended mass, including its frame, connectors and attached hardware. The enclosure sizing guide covers the separate space needed for wiring and service access; isolation travel must be added explicitly.
Build isolation that works in every direction
Support the fragile assembly on a sufficiently rigid internal carrier when its design permits. Keep the sensitive components fixed to that carrier rather than changing their established internal relationships.
Select mounts for the actual supported load and displacement in compression, tension and shear. Obtain dynamic stiffness, damping, allowable travel and shock suitability from the mount manufacturer.
Control translation and rotation together
Place supports so their combined stiffness acts near the suspended assembly's center of mass where practical. Large offsets can turn a translational impact into a rocking motion.
A widely spaced support pattern can improve rotational restraint. However, adding mounts also increases total stiffness unless each mount is selected accordingly.
Evaluate all three translations and all three rotations. Corner movement can exceed movement at the center of mass, especially with a tall or offset payload.
If the complete enclosure changes orientation during testing, gravity changes direction relative to the mounts. Check resulting preload and sag for each orientation, not only the installed position.
Do not treat damping as a universal cure
Very light damping can allow excessive motion and repeated oscillation. Excessive damping can transmit more of a short impact pulse.
Choose stiffness and damping against the measured input pulse, payload fragility and available travel. A vibration-isolation frequency ratio alone does not establish drop-test performance.
For a sensitive assembly, compare candidate designs using transient analysis or a qualified isolation specialist. Validate the selected design physically before accepting the calculation.
Keep every service connection compliant
Provide cable loops that accommodate the complete motion envelope without pulling connectors or entering pinch points. Check flexible thermal connections and protective bonding conductors as carefully as signal wiring.
Do not remove required bonding to improve isolation. The enclosure grounding and bonding guide explains the electrical requirements that remain in force.
Keep moving cables clear of fixed glands and cutout edges. Use the cutouts, glands and windows guide for those stationary enclosure interfaces.
Validate with measured shock pulses
Agree on the test conditions before comparing designs. Drop height alone does not define an equivalent test.
Record the impact surface, release method, enclosure mass, payload configuration and required orientations. Include the required number and sequence of impacts from the governing test specification.
Use synchronized, calibrated accelerometers on the enclosure near the support structure and on the payload carrier. Triaxial measurements help identify cross-axis response and rotation-related differences.
Select sensor range, mounting, bandwidth, sampling rate and anti-alias filtering for the expected pulse. A clipped signal cannot establish that the payload stayed below its limit. Loose sensor mounting can introduce misleading resonances.
Keep processing consistent between baseline and modified tests. Report filtering and distinguish measured acceleration from any gravity correction used in interpretation.
Evaluate more than a single peak:
- Input and payload peak acceleration in each measured direction.
- Pulse duration, shape and significant secondary impacts.
- Relative displacement and remaining hard-stop clearance.
- Shock response spectrum when the fragility assessment requires it.
- Functional performance and evidence of permanent mechanical change.
A failure observed at one peak value does not establish a universal safe acceleration limit. Pulse duration, direction and repeated exposure can change the result.
Where fragility is unknown, establish an agreed characterization program using suitable test assemblies and conservative limits. Increase severity under controlled conditions rather than repeatedly dropping the final assembly without a measurement plan.
Accept the design only when the required tests demonstrate both functional survival and adequate travel. Inspect mounts, attachments and sensitive components between test stages.
Stop the failure returning
Preserve the validated mechanical configuration in the production drawing and assembly instructions. Small substitutions can materially change a shock-isolation system.
Specify mount identity, orientation, fastener details, tightening requirements and any preload settings. Record cable routing and clearance dimensions rather than relying on assembly photographs alone.
Control changes to payload mass and center of mass. A heavier replacement component changes mount loading, natural frequency and available sag clearance.
Include periodic inspection for elastomer aging, compression set, loose attachments and cable stiffening. Use the mount manufacturer's environmental limits for temperature, oils and cleaning chemicals.
If a progressive stop or restraint is fitted, confirm its gap after assembly. It must not become a rigid shortcut during the qualified operating motion.
Maintain the shock-test record alongside the enclosure revision. Reassess the design after changes to mounts, impact-contact features, payload layout or service connections.
When the enclosure needs more room
Re-specify the enclosure when the required stopping travel cannot fit with adequate reserve. Also reconsider the shell when its supports flex excessively or local distortion damages sealing interfaces.
A stronger shell is not automatically a better shock protector. It may preserve its shape while transmitting a sharper pulse to the internal mounts.
We build custom enclosures with customer-defined material, width, height, depth and cutouts. Define the isolation envelope before fixing those dimensions, rather than choosing a box around the stationary payload alone.
Send the suspended mass, center-of-mass location, proposed support coordinates and required clearance envelope with the enclosure request. Include measured input pulses, required impact orientations and the payload acceptance criteria.
The enclosure builder can capture the enclosure configuration for a quote and drawing approval. Isolation-system design and shock qualification still require their own engineering evidence.
Specify environmental protection separately from shock performance. Our enclosure ratings are manufacturer ratings; the enclosures are not UL listed. A successful drop test does not establish an ingress rating, and an ingress rating does not establish protection for fragile internals.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 6 × 6 × 4 in | 304 stainless | 6 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 8 × 6 × 4 in | 304 stainless | 8 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 10 × 8 × 4 in | 304 stainless | 10 x 8 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Will a thicker enclosure stop fragile internals failing a drop test?
Not necessarily. A stronger enclosure can remain intact while transmitting a severe acceleration pulse to its contents. First determine whether failure comes from shell deformation, direct shock transmission, exhausted isolation travel or local resonance. Each mechanism needs a different corrective action.
How much isolation travel does a 4 in drop require?
The required travel depends on the payload's allowable shock response and the measured enclosure pulse. For an idealized stop neglecting gravity, 0.20 in [5.1 mm] of stopping distance corresponds to 20 g average deceleration after a 4 in drop. Actual peaks and required clearance can be higher.
Can one mount arrangement protect the payload in every direction?
It can, but only when its stiffness, damping, load capacity and travel suit every required direction. Check compression, tension, shear and rotational motion. Cable bundles, bonding conductors and thermal connections must accommodate that movement without creating rigid bypass paths.
Is staying below a measured peak acceleration enough to pass?
No. A peak value must be interpreted with pulse duration, direction, filtering and the payload's structural response. Confirm functional survival, inspect for permanent damage and verify remaining travel. Sensitive resonant assemblies may also need shock response spectrum limits and repeated-impact evaluation.
