Machinedesign 5821 Locking Device Gerwah 0

Engineering roundtable: Proper use of locking devices in motion applications

July 1, 2005
Locking devices connect, or power transmission components to rotating shafts. In this report, we polled locking-device experts for their advice on optimizing lifetime and ease of use. Here are the responses, which we believe you'll find most helpful.

Locking devices connect power-transmission components to rotating shafts. In this report, we polled locking-device experts for their advice on optimizing lifetime and ease of use. Here are the responses, which we believe you'll find most helpful.

What particular design and construction features in locking devices contribute to higher productivity, and why?

Mark/B-Loc: Keyless locking devices employ high-strength fasteners and alloy steels, which provide true zero-backlash power transmission connections. Most self-lock, making them durable in demanding motion control applications such as robotics, servo-driven timing pulleys, and indexing tables. They can be removed and reinstalled using only a torque wrench — allowing quick and easy access to mounted bearings, seals, and other wear parts. In precision-critical motion applications, keyless locking devices guarantee 100% repeatable accuracy for the equipment's life.

Doug/Zero-Max: Keyless shafts and single screw adjustment designs contribute to higher productivity. Mounted components such as hubs, gears, and sprockets rotate freely or rephrase on the shaft. A single-screw design provides quick, unlimited locking and unlocking of the bushing, saves time, and decreases component wear. Locking devices ensure proper balance and very little runout; their unique design also accommodates smaller-diameter shafts without keyways.

Andrew/Whittet-Higgins: Three design features contribute to higher productivity:

  • One-piece mounting
  • Torque/load measuring capabilities that ensure sufficient tightness
  • Precise manufacturing for smooth contact with other components

Terry/U.S. Tsubaki: Keyless devices eliminate machining expenses, downtime, and maintenance because they easily slide into position. Cut keyways, splines, and grooves reduce shaft strength, since stress is concentrated at the cut. Therefore, a keyless device leaves the shaft intact and operates longer, especially in high-torque or hollow-shaft applications.

Slight discrepancies in fit cause backlash and fretting, which worsen over time and result in premature equipment failure. A keyless friction bond provides 360° shaft-hub contact and stays tight under shock and reversing loads.

Blair/Ruland: Most locking devices contain standard bore sizes and nonstandard hub requirements. Understanding that hubs are generally machined to match the locking device reduces rework and smoo

thes the development process. Minimum hub thickness is also critical and depends on locking device outer diameter and surface pressure. Hubs should be thicker than this calculated value to prevent excess stress and early failure.

Jeff/Lovejoy: Keyless locking devices offer a low-cost interference fit — optimum for high-torque, pulsating, or reversing load applications. This fit eliminates keys/keyways, fretting, corrosion, reduces replacements for damaged parts, curbs installation costs, and incorporates zero-backlash. Pre-assembly tolerances accommodate easy assembly or removal of mounted parts, without damaging the shaft or hub.

Eric/Fenner: Most keyless designs have two concentric rings with tapered faces drawn together through a threaded device. When pulled together, pressure builds inward on the inner ring and outward on the outer ring — locking the hub to the shaft. Machining a keyway takes significant time and weakens the shaft. During frequent start and stop applications and heavy vibrations, a wallowed bore and damaged key can result.

Keyless bushings can be positioned radially, allowing the teeth of each sheave to properly align with the teeth of the belt. Furthermore, a keyless locking device may be positioned along the length of a shaft, aligning both pulleys and sheaves for regular and synchronous drives. Improperly aligned sheaves cause premature belt failure and frequent downtimes.

FEA plot of von Mises stresses courtesy R+W.

What can designers do to ensure higher productivity from the locking devices they place in machines?

Mark/B-Loc: Locking devices that transmit peak, dynamic loads maximize productivity. When this is not possible — as is frequently the case in short-cycle applications with shock or reversing loads — designers should consider nominal operating loads and an appropriate safety factor. Mounted components (timing pulleys, lever arm hubs, pinion gears) must possess sufficient size and strength to accommodate a keyless locking device. In addition, proper installation ensures zero-backlash, maintenance-free operation.

Doug/ZeroMax: Designers must identify potential problems in connections and mounting modules. Radial loads, mounting frequency, temperature variations, space constraints, shaft keyways, and tapers all influence a system's productivity.

Andrew/Whittet-Higgins: Applications that address assembly locking access, loading or space constraints, and resistance to corrosive chemicals and temperatures ensure higher productivity. In addition, open, cooperative discussions between designers and manufacturers during development prevent assembly problems.

Terry/U.S. Tsubaki: Locking device costs must balance against torque, load, temperature, moisture, and frequent component removal. New designs and materials improve productivity in challenging conditions such as corrosion-resistant food applications and high-precision indexing machines.

Keyless locking devices are unaffected by torsional load reversal or impact on keys and keyway connections. They handle both high torque and thrust while increasing shaft strength. In addition, smaller diameters and bearings can be used, thereby reducing inertia, cost, and size of the final design.

Blair/Ruland: It's important to look at where and how the locking device is used, and if adjustments will cause problems. Sometimes it is best to perform adjustments from another section of the machine, since locking devices are difficult to remove and replace.

Jeff/Lovejoy: Designers should utilize the following features:

  • High strength — Keyless locking devices enable use of a smaller diameter shaft, since a cut keyway does not weaken the shaft.
  • Position control — This improves timing or positioning adjustments during assembly.
  • Backlash-free — Locking devices hold firm in high-shock or reversing load applications and eliminate traditional problems with keyed/splined connections.
  • Design ease — Straight bores and outer diameters utilize standard shafts and hubs and eliminate machining expensive tapers, keyways, and splines.
  • Proper selection and sizing — Understanding the differences between models aids in proper selection. Material strength of the hubs ensures the correct minimum thickness to avoid failure.

Eric/Fenner: Single-nut keyless bushings self-contain all components and deliver them as one assembly. Customized lengths meet user needs, and customized installation nuts enable manual tightening through a knob (versus a hex nut). Most importantly, the unit locks itself to the shaft through one turn of the nut.

Locking-device application photo courtesy ZeroMax.

As torque transmission requirements increase, so must installation torque of the nut, requiring the use of a multi-screw. The installation torque locking a device in place is shared among several screws. As torque increases, the screw handles larger loads and eases installation.

What can end users do to ensure higher productivity from the locking devices on their machines?

Mark/B-Loc: Applying grease and/or a cover plate over an installed, keyless locking device protects jacking holes and eases removal. Disassembled keyless locking devices can be reused after thoroughly cleaning and relubricating. Ultimately, productivity and uptime increase compared to machines with keys, setscrews, and/or splined connections.

Doug/ZeroMax: Locking devices provide fast and frequent mounting/dismounting with only one radial-adjusting screw, thereby saving space and providing accurate mounting without axial movement. This uniform pressure against the shaft and hub prevents surface damage and uses a smaller diameter shaft for superior torque. Devices also operate in temperature ranges from -22° to 180°F.

Andrew/Whittet-Higgins: The quality of all components determines the precision of assembly equipment. In addition, the manufacturer's education and training is crucial.

Terry/U.S. Tsubaki: Early discussion with the manufacturer's engineering team can expose potential problems and reduce design costs for extended productivity. Keyless devices decrease maintenance and downtime, position more quickly without aligning keyways, and suit frequently adjusted or periodically removed applications. Locking screws do not require retightening after long production runs and can disassemble, reassemble, and return to original fit using one tool.

Blair/Ruland: During service, locking devices must be inspected for signs of slipping, and clamping screws must be tightened to the recommended seating torque. This involves tightening opposing screws and working around the locking device to reduce misalignment and maximize holding power.

Jeff/Lovejoy: Keyless locking devices easily move to reduce other components' wear or loosen and reposition for timing applications. Cover plates and packing grease lengthen life in dirt and corrosive environments. In addition, a torque wrench should be used to tighten locking screws. One note of caution: over-tightening strips screws or bursts hubs.

Eric/Fenner: Single-nut keyless bushings lock components to the shaft with a single nut — helpful in assembly line applications and in frequent component removals and reinstallations. In place of steel, electroless nickel and stainless-steel plating protect against washdowns and hostile environments. However, steel locking devices withstand wide temperature ranges.

What are some common shortcomings in locking device design or construction, and how do they affect productivity?

Andy/R+W: Keyway wear and shaft slipping are two common failures. Torsional loads concentrate stress inside the keyway's edges during intermittent or frequently reversing runs, causing backlash and deterioration of the clamping element. Shaft slipping occurs on tangentially clamping devices with a smooth, keyless bore. Lightly oiled shafting, a slightly rough-surface finish (60 rms), and an overall-fit tolerance ranging from 0.01 to 0.05 mm are advised. The tangentially clamping hub evenly distributes material stress around the clamp's circumference, making it a better solution.

Bill/Gerwah: Over-tightened locking screws, or those exposed to excessive bending, elongate or shear — becoming incapable of transmitting their rated capacity. Incorrect diameters or improper materials cause hub deformations and cracking; shafts and locking devices also damage when system torque overcomes the holding force — due to poor design and/or installation. Further, a successful application requires skilled machinists so that torque capacity is not compromised.

Celestino/Tollok: Incorrect selections and insufficient hub dimensions cause failures. Loads affecting the locking assembly during operation, the correct service factor, and the hub's size must all be considered. A knowledgeable, experienced supplier is crucial for avoiding failures and weak points in the locking assemblies.

What are some of the common mistakes designers make when selecting and applying locking devices, and how do these mistakes affect productivity?

Andy/R+W: Jack screws assist locking device removal by separating a conical sleeve and its bushing in a taper-lock mechanism. Alternatively, a pin installed across a counter bore serves as a screw's socket head; when the screw backs out of the clamp, its head engages the pin and opens the clamping mechanism.

Bill/Gerwah: Lost productivity and added cost result when designers overlook details such as speed, material strength, wall thickness, hub length, environment, and access for tools. Unaddressed environmental concerns during selection later lead to corrosion that complicates removal. Another mistake is applying nickel-plated or stainless-steel locking devices to corrosive applications and assuming the transmittable torque capacity is the same as carbon-steel products. Application torque increases when locking devices are selected based on the motor or gearbox's rated torque capacity, rather than evaluating dynamic forces — such as shock loads, start-up torque, bending moment, axial and radial loads, and torque reversals. In addition, designers must reduce shaft diameter when applying locking devices.

Celestino/Tollok: The most frequent mistake is disregarding all the loads affecting a locking assembly during operation. Sometimes, only the rated torque is considered, rather than the peak load — or the bending moment may not be taken into account. In other cases, the hub's external diameter is sized incorrectly, causing elastic failure in the material, lower performance, or a stopped machine.

What are some of the common mistakes end users make with regard to locking devices, and in what way do these mistakes affect productivity?

Andy/R+W: Commonly, end users overtighten clamping screws. Specified tightening torques optimize distribution of clamping forces and minimize component failure.

Bill/Gerwah: End users often misunderstand the operating principal, which leads to damaged locking devices and mounted components. Grease applied to the driven shaft causes a proportionally lower torque capacity, resulting in slippage and component damage. Improperly used adhesives on the screw threads hinder removal. Mounted components that are erroneously replaced destroy the locking device and mounted component.

Celestino/Tollok: The most frequent mistake end users make is disregarding installation instructions; for example, tightening the screw without using a torque wrench. This simple tool enables quick and exact tightening and prevents locking assemblies from slipping, the machine from stopping, and lost productivity.

Meet the locking-device experts

Celestino Accetturo
Engineering Manager, Tollok North America Inc. • Auburn, Ga. • (770) 867-4848

Terry Ando
Senior Design and Applications Engineer, U.S. Tsubaki Inc. • Wheeling, Ill. • (800) 323-7790

Andrew Brown
Whittett-Higgins Co., Central Falls, R.I. • (401) 728-0700

Bill Dair
Gerwah Drive Components, Forest Park, Ga. • (800) 211-3968

Jeff Jewel
Vice President, Lovejoy Inc. • Downers Grove, Ill. • (630) 852-0500

Andy Lechner
R+W America L.P. • Bensenville, Ill. • (630) 521-9911

Doug Moore
Zero-Max Inc., Plymouth, Minn. • (800) 533-1731

Blair Morad
Product Engineer, Ruland Mfg. Co. Inc. • Wheeling, Ill. • (508) 485-1000

Eric Mosser
Product Development Engineer, Fenner Drives USA • Manheim, Pa. • (800) 243-3374

Mark Rabens
Vice President, B-Loc Corp. • Monroe, N.Y. • (800) 865-7756

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