> Umbra Group – Screwshaft and ballnut from aerospace experience

Italy, Foligno – Special alloy and heat treatment for the consistent enhancement of the ballscrew fatigue properties  

Aerospace experiences gained in over 35 years allowed Umbra to develop a special combination of alloy and heat treatment. This technical development for both screwshaft and ballnut which leads to a consistent enhancement of the ballscrew fatigue properties (with lifetime twice that of the standard configuration).

Passion, research and teamwork are the ingredients of Umbra Group.

A component by Umbra Group for machining centers.

From special alloy and heat treatment, the short stroke ballscrews

If the required stroke is short compared to the thread portion needed for the rolling elements, the recirculation system can be incorporated in the translating shaft.

The balls move along one single turn and are brought back to their initial position through either a deflector located in a cavity inside the screwshaft or machining the return path directly into the screwshaft – the latter being a more reliable configuration because of the lower number of parts.

Such a solution minimizes the ballscrew envelope (i.e. smaller diameter and shorter length) and components; moreover, ball recirculation incorporated in the translating shaft allows a cylindrical geometry (i.e. a rod that can be also be the output rod), with consequent lower drag torque and higher sealing capacity.

The backlash of ballscrew assy can be set at a “minimum functional” value, i.e. the lowest possible to allow free fall of the screwshaft under its own weight (in the range of 0.01 mm).

A solution by Umbra Group used for turning and milling machines.

Integrated architecture for less components

Complex assemblies like EMA (Umbra acronym for Electro Mechanical Actuator)  are made by high number of components. One of Umbra advantages is represented by the integrated architecture, which allows a consistentd reduction of these components with benefits for both cost and manufacturability.

Examples are represented by the bearing raceways integrated on the ballnut shoulders and the rotating ballnut configuration.

Health Monitoring based on Prognostic and Diagnostic

Umbra started Health Monitoring activity back in 2008, basing it on both Prognostic and Diagnostic. Both disciplines are based on the strong relationship between mechanical failure and critical parameters (e.g. current, vibration, temperature).

Prognostic may identify premature fault of the system allowing a preventive action in order to avoid the failure, while Diagnostic identifies in detail the faulty component(s) and the related failure mode.

Ball bearing by Umbra Group for rotary tables.

Depending on the system complexity, the following approaches may be used:

1) Model-based approach

Suited for low complexity systems. The system is represented by a simplified mathematical model, specific formulas describing the phenomenon.

The monitoring of critical parameters allows the estimation of the health status of the system. Umbra gained significant experience in induced defects tests.

2) Model-free approach

Suited for high complexity systems, like for instance EMA (Umbra acronym for Electro Mechanical Actuator). Even if input and output data are related, the analytical function which maps them cannot be determined in advance.

Nevertheless, selecting the critical process parameters at the beginning and monitoring their variations allows, through a dedicated algorithm, to determine the potential failure rise.

Wear and life expectation

All the above provides a reliable indication of wear and life expectation of the mechanical components.

Umbra has currently partnerships with market leaders at forefront of prognostic and diagnostic strategy.

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