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ASME B107.7-1978 — Wrenches, Splined Socket, Square Drive, High Strength, Thin Wall; Box Wrench, High Strength, Thin Wall (Inch Series)
🔧 ASME B107.7-1978 is the ASME B107 hand-tools standard dedicated to wrenches for high-strength, high-temperature splined fasteners, covering two product families: splined socket wrenches with square drive (high-strength, thin-wall) and splined box wrenches (high-strength, thin-wall), all dimensioned in inch units. Approved by the American National Standards Institute (ANSI) in 1978 and designated ANSI B107.7-1978, it addresses fasteners whose external wrenching element is a spline per MS33787 — the configuration typical of aircraft engines, gas turbines, and other high-strength, high-temperature bolted joints.
💡 Spline drive differs from hex drive: torque is transmitted through multiple evenly spaced teeth, giving more contact area and, combined with a thin-wall design, higher torque capacity in a given joint envelope — hence its use on high-strength bolts.
1. Scope and Classification 📌
Section 1 covers the complete general and dimensional data for detachable socket wrenches and box wrenches for high-strength and high-temperature splined fasteners. Section 2 classifies the products:
- Class 1 — Splined socket wrench: square drive; Style A regular length and Style B long length.
- Class 2 — Splined box wrench: double head, 15° offset each end.
Referenced documents include ANSI B107.1 (hand socket wrenches, inch series), ANSI B107.4-1973 (driving and spindle ends for portable, hand, air, and electric tools), ASTM E18 (Rockwell hardness), FED-STD-346 (wrench opening gages), MS33787 (external spline wrenching element dimensions), and ANSI B46.1 (surface texture). The spline opening is designed to fit the external spline of MS33787 and is accepted/rejected by the GO/NO GO gages of FED-STD-346.
⚠️ The socket opening is designed around the MS33787 external spline element and is not interchangeable with ordinary hex or 12-point sockets. Using a conventional socket on a splined fastener damages the teeth and defeats torque transfer.
2. Technical Requirements and Specifications 📏
Section 4 covers materials, marking, hardness, test loads, wrench opening, surface roughness, bolt clearance holes, and countersinking; Section 5 defines test procedures. Key parameters are summarized below:
| Item | Requirement | Clause |
|---|---|---|
| Hardness | Hardened and tempered, Rockwell C 48–52 | 4.4 |
| Wrench opening (spline) | Per Table 1 spline dimensions; GO gage passes, NO GO gage rejects | 4.6 |
| Test loads | Withstand test load and endurance torque per tables; no failure or permanent deformation | 4.5 |
| Surface finish | ≥50% of outer longitudinal surface bright, 30 microinch (0.76 µm) max; remainder 150 microinch (3.81 µm) | 4.7 |
| Bolt clearance hole | All sockets; per Table 2; long-length ≥70% of overall length from wrench-end face | 4.8 |
| Spline opening countersink | 120°–150° included angle; minimum diameter equal to major spline dimension | 4.10 |
The Standard also requires an internal bolt clearance hole in every socket (diameter per Table 2): regular-length sockets extend the hole from the base of the spline opening to the base of the square drive; long-length sockets from the wrench-end face for not less than 70% of overall length, with a through hole of minimum diameter equal to 50% of the square-drive size or the clearance-hole diameter, whichever is smaller, so bolt shanks can pass through. All female drive openings shall be broached, punched, formed, or machined smooth and well defined per ANSI B107.4 square-drive specifications.
💡 The narrow 48–52 HRC window is a signature of this Standard: enough hardness for the wear and bearing demands of high-strength joints, yet not so hard that the tool becomes brittle — markedly different from ordinary hand tools.
3. Test Procedures 🔬
- Hardness test: per ASTM E18 with a diamond penetrator and 150 kg major load; surfaces suitably prepared and plating removed before testing.
- Load test: a suitable torque machine applies the test load and endurance torque of Section 4.5 for 10 seconds, then removes it; no failure or service-affecting permanent deformation is permitted.
- Dimensional and gage checks: the spline opening is judged with FED-STD-346 GO/NO GO gages; other dimensions per Figs. 1–2 and the tables.
🚨 Load tests must hold the specified 10-second dwell and controlled application; shock loading, overruns, or non-standard fixturing can let a “verified” tool fail under real high loads — and fracture of high-strength spline tooling in aerospace/power duty is particularly serious.
4. Engineering Practice Essentials 🛠️
- 🔍 Selection: confirm the fastener is an MS33787-compliant spline element, then pick the socket/box wrench whose opening matches Table 1.
- 🔧 Torque control: high-strength fasteners are torque-sensitive — use a calibrated torque wrench and staged loading, never a single hard pull.
- 📊 Thin-wall care: thin walls transmit more torque in less space but are more sensitive to knocks and side loads; protect during transport and storage.
- 📋 Marking: tools must be clearly and permanently marked with the manufacturer’s name/trademark and the opening size; verify marks at receiving inspection for traceability.
Because this Standard targets aerospace- and power-oriented joints, shop-floor discipline matters. Locking and safety-wired hardware must never be loosened with an incorrectly sized socket; the tool must seat fully so all spline teeth share the load. When sockets are stored, keep them separated from standard hex tools and label trays by spline size so that mis-selection cannot happen in a hurry. Regular dimensional inspection with FED-STD-346 setting gages catches worn openings before they round off spline teeth, protecting both the tooling investment and the high-value fasteners themselves.
5. Frequently Asked Questions ❓
❓ Q: What is a “spline wrenching element”?
A: It is the external spline on a bolt/nut head, standardized by MS33787. With many teeth and high torque capacity, it suits high-strength, high-temperature joints such as aircraft engines; B107.7 sockets are designed around it.
A: It is the external spline on a bolt/nut head, standardized by MS33787. With many teeth and high torque capacity, it suits high-strength, high-temperature joints such as aircraft engines; B107.7 sockets are designed around it.
❓ Q: Can an ordinary socket turn a splined fastener?
A: No. The splined socket opening is shaped and pitched for the MS33787 external spline — completely different from hex/12-point; mixing slips and damages the teeth.
A: No. The splined socket opening is shaped and pitched for the MS33787 external spline — completely different from hex/12-point; mixing slips and damages the teeth.
🔍 Q: Why is the hardness window so narrow at 48–52 HRC?
A: High-strength spline joints demand adequate compressive and wear hardness while avoiding brittle fracture; the narrow window ensures consistent performance and repeatability across production lots.
A: High-strength spline joints demand adequate compressive and wear hardness while avoiding brittle fracture; the narrow window ensures consistent performance and repeatability across production lots.
🔧 Q: What is the purpose of the bolt clearance hole?
A: It lets the bolt shank pass into the socket so the tool seats fully over the nut or bolt head, ensuring complete spline engagement — essential for long bolts and long-length sockets.
A: It lets the bolt shank pass into the socket so the tool seats fully over the nut or bolt head, ensuring complete spline engagement — essential for long bolts and long-length sockets.
