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ASME PALD 5-1987 — Mechanical Screw Jacks Safety Standard
⚙️ ASME/ANSI PALD-5-1987 is the “Mechanical Screw Jacks” volume of the ASME Safety Standard for Portable Automotive Lifting Devices. Unlike hydraulic jacks, a mechanical screw jack converts the rotary motion of a crank handle into vertical lift through a screw thread pair (typically trapezoidal or buttress threads); it is simple, contains no hydraulic fluid, and is well suited to on-board vehicle use. This Standard provides a complete safety framework for the design, marking, testing, operation, inspection, and maintenance of such jacks, and is a key reference for vehicle tool kits and service operations.
💡 The subject is explicit: this volume applies to “self-contained” mechanical screw jacks — units with the lifting mechanism and operating handle built in, needing no external hydraulic or power source — used to lift one wheel of a vehicle clear of the ground for emergency tire changing.
1. Scope and Technical Parameters 📌
Section 1 defines the scope: this volume applies to self-contained mechanical screw jacks designed for lifting one wheel clear of the ground for emergency tire changing on vehicles up to 11,000 lb (4,990 kg) gross vehicle weight. The purpose clause (para. 1.2) states that the volume establishes guidelines on construction, care, and use, providing a basis for understanding among manufacturers, distributors, and operators.
| Parameter / Item | Typical Requirement / Value | Notes |
|---|---|---|
| Applicable vehicles | GVW ≤ 11,000 lb (4,990 kg) | Passenger cars, light trucks — roadside tire changing |
| Rated capacity | As marked by the manufacturer (typically 1–3 short tons) | Maximum operating load the jack is designed to support over its lift range |
| Lifting mechanism | Screw thread drive (thread pair + crank/pawl) | Mechanically self-locking; no hydraulic fluid |
| Lift objective | One wheel clear of the ground | Emergency tire changing only — not whole-vehicle support |
| Follow-up support | Vehicle must be placed on support stands after lifting | See PALD-4 (Vehicle Support Stands) |
⚠️ Two “nevers” to remember: first, a screw jack must never support a vehicle for extended periods — place the vehicle on support stands (PALD-4) immediately after lifting; second, never use this on-board jack to lift vehicles or loads beyond its rated capacity or designed purpose.
2. Design Qualification Testing 📋
Section 5 requires that every design, or design change affecting safety, pass design qualification testing:
- Proof load test: a load greater than the rated capacity (typically 150% of rated capacity) is applied centrally to the saddle to confirm structural integrity.
- Off-center load test: simulates loads not perfectly centered during actual tire changing, verifying the jack’s stable load-carrying capability under off-center loading.
- Acceptance: after testing there must be no functional damage — no permanent deformation causing loss of load or excursions beyond the qualification limits; threads, saddle, base, and crank mechanism must remain sound.
🚨 The “self-locking” behavior of a mechanical screw jack relies on thread friction: once the threads are worn, bent, or improperly lubricated, self-locking degrades and the load can settle on its own. Any jack found with thread damage, deformation, or stiff operation must be removed from service immediately.
3. Product Marking and Warnings 🏷️
Section 4 requires clear product identification:
- Rated capacity: marked prominently (typically on the base or housing) by casting imprint, metal stamp, or durable materials.
- Manufacturer identification: identifying mark(s) of the original manufacturer or supplier, with the manufacturer able to identify the date of manufacture.
- Warning label: durably affixed where the operator can see it, containing the word WARNING as a heading, a statement of the hazard, what to do to avoid or reduce it, and a reminder to study and follow all instructions. Typical warnings include “do not overload,” “place the load on the center of the saddle,” and “use on hard, level ground.”
4. Operation, Maintenance, and Inspection Essentials 🛠️
Sections 6 and 7 address the specifics of screw jack use; in practice, follow these points:
- 🔍 Pre-use inspection: check the threads for burrs or wear, verify the crank mechanism is sound, and confirm the saddle is not deformed; confirm smooth, self-locking rotation.
- 🔧 Correct placement: set the jack on hard, level ground, align the saddle with the manufacturer-designated lift point (frame rail or subframe), and never jack on the oil pan, exhaust, or other thin-walled parts.
- 📐 Center the load: the vehicle lift point must sit on the center of the saddle; keep any body part out from under the vehicle during the tire change.
- 📋 Periodic lubrication and inspection: lubricate threads and pivots per the manufacturer’s manual, check wear, and keep records.
- 📊 After use: once the tire is changed, transfer the load to support stands, then crank the jack down and stow it.
5. Frequently Asked Questions ❓
❓ Q: How do I choose between a screw jack and a hydraulic jack?
A: Screw jacks are simple, leak-free, and easy to carry — ideal for roadside tire changing; hydraulic jacks lift smoothly and with less effort, suiting workshop maintenance. Either way, the rated capacity must exceed the actual load.
A: Screw jacks are simple, leak-free, and easy to carry — ideal for roadside tire changing; hydraulic jacks lift smoothly and with less effort, suiting workshop maintenance. Either way, the rated capacity must exceed the actual load.
🔍 Q: Why does the Standard limit the jack to lifting one wheel only?
A: The on-board screw jacks in scope are designed and tested for the “one wheel clear, roadside tire change” duty; their capacity and stability are optimized for that scenario. Raising a whole axle or vehicle needs higher-capacity shop equipment (e.g., PALD-10 service jacks); mixing the two overworks the jack.
A: The on-board screw jacks in scope are designed and tested for the “one wheel clear, roadside tire change” duty; their capacity and stability are optimized for that scenario. Raising a whole axle or vehicle needs higher-capacity shop equipment (e.g., PALD-10 service jacks); mixing the two overworks the jack.
❓ Q: Is the proof load test mandatory?
A: Design qualification testing is the manufacturer’s responsibility and must be performed for every design or safety-relevant design change. At incoming inspection, purchasers should verify that the product conforms to PALD-5 and request test and conformity documents.
A: Design qualification testing is the manufacturer’s responsibility and must be performed for every design or safety-relevant design change. At incoming inspection, purchasers should verify that the product conforms to PALD-5 and request test and conformity documents.
❓ Q: Does the vehicle still need support stands after the screw jack has lifted it?
A: Yes. The jack only raises and lowers; it should not carry the load for long. During tire changes, brake work, or other service, the vehicle must be supported at preset heights by pairs of vehicle support stands (PALD-4).
A: Yes. The jack only raises and lowers; it should not carry the load for long. During tire changes, brake work, or other service, the vehicle must be supported at preset heights by pairs of vehicle support stands (PALD-4).
