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Altra Industrial Motion has received its first prototype order from a European helicopter manufacturer for sprag retainer assemblies.

Formsprag sprag retainer assemblies offer numerous features that are designed to boost helicopter performance and lifespan, as well as assure safety of operation.

With increasing speeds and power placing demands on freewheels that cannot be met by roller type clutches, Formsprag sprag retainer assemblies address this need by offering high-torque density freewheels with overrunning speeds of up to 38,000rpm.

Formsprag sprag retainer assemblies are regularly used in the driveline gearboxes of multi-engine helicopters to allow twinning of the engines for efficient power sharing and synchronisation or disengagement of one engine.

Formsprag freewheels offer a number of design features that are said to make them ideal for helicopter applications, as well as many fixed-wing aerospace tasks.

The Formsprag sprag retainer assembly design permits each sprag to have free and independent action.

This independence allows each sprag to adapt to variations in annular space (eccentricities) so that when the clutch is engaged, the load is proportionally shared among all sprags, eliminating the possibility of clutch damage resulting from the entire load being absorbed by just a few sprags.

This ensures dependable performance without compromising size, weight or cost.

The Formsprag positive continuous engagement (PCE) design prevents sprag rollover during torque overload conditions – a vital safety feature in aerospace applications.

The PCE retainer was developed specifically to meet the demands of high-performance helicopter and aircraft applications.

It provides reliable performance even under extremes of torsional vibration and severe transient overload, and is suitable for confined space applications where minimal weight is critical.

For extended operational life in demanding applications, Formsprag freewheels are manufactured using the company’s Formchrome process, which diffuses chromium carbide into the working surfaces of the sprag, increasing its surface hardness to deliver wear protection.

Through a variety of specifications matched to the application requirements, Formchrome can be utilised to increase the surface hardness of the base metal – SAE 52100 steel – from 800 Knoop (approximately 63 Rc) to around 1,300 Knoop (equivalent to 75 – 80 Rc), or even 2,000 Knoop (equivalent to 90 – 95 Rc).

Subsequent heat treatment ensures a hard sprag core to support the chrome surface.

In operation, springs energise the sprags into position for instantaneous engagement with no backlash.

Contracting, expanding and torsional springs have also been developed by Formsprag.

For high-speed overrunning applications that are typical in helicopter systems such as starters, Formsprag’s centrifugal throw-out (C/T) design locates the centre of gravity of the sprag relative to its contact point on the outer sprag diameter, such that when the outer race is overrunning the centrifugal force on the sprags overcomes the energising force of the springs.

This causes the sprags to completely lift off the inner race, with the advantage that there is no rubbing contact during operation, further extending the lifespan of the assembly.

Formsprag has a C/T system with close control over disengagement and re-engagement speeds.

Altra has established systems and procedures that support the aerospace industry; these include manufacture to ISO 9001:2008 and aerospace-specific AS9100 rev B quality standards.

The company also holds a number of customer source inspection delegation certifications for global aerospace customers and is committed to continuous improvement through the Altra Business System (ABS).

Altra Industrial Motion

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