Why Must Some Shafts Be Plunge Ground? The Problem of Helical Lead Marks

luglio 29, 2026

When reviewing the drawing of a shaft designed to work with rotary shaft seals, it is not uncommon to find specifications such as Plunge Grinding, No Helical Lead, D = 0° ± 0.05°, or Dt ≤ 0.3 µm. At first glance, these requirements may appear to be simple surface finish specifications.

In reality, their purpose is entirely different. They are not intended to produce a smoother surface. They are intended to produce a surface that cannot transport lubricant. This is a little-known aspect of shaft manufacturing, yet it can determine whether a sealing system performs reliably throughout its service life or develops premature oil leakage.

When a machined surface becomes a micro pump

During conventional cylindrical grinding, the grinding wheel moves longitudinally along the shaft while removing material. This axial feed leaves a series of microscopic grooves on the surface. Although these grooves are only a few tenths of a micron deep, together they form a helical pattern. When the shaft rotates, this microscopic helix can behave like the thread of a screw. As a result, lubricant is slowly but continuously pumped either inward or outward, depending on the direction of rotation and the orientation of the helix.

The effect is invisible to the naked eye. Nevertheless, it can be sufficient to compromise the sealing performance of an otherwise perfectly designed radial shaft seal.

Why surface roughness alone is not enough

Surface quality is usually evaluated through parameters such as Ra, Rz and Rt. These values are essential because they describe the height of the surface irregularities. However, they say nothing about their orientation. Two shafts may exhibit exactly the same Ra value while behaving completely differently in service. One surface may have predominantly circumferential machining marks and provide excellent sealing performance.

The other may contain microscopic helical grooves capable of pumping lubricant across the seal lip. From a conventional roughness perspective, both surfaces appear identical. From a functional standpoint, they are not. This is why many manufacturers specify not only the required roughness values but also the grinding process itself.

Why plunge grinding solves the problem

Plunge grinding is specifically intended to eliminate this phenomenon. Unlike longitudinal grinding, the grinding wheel feeds radially into the workpiece without significant axial movement. The resulting machining marks become predominantly circumferential rather than helical. This dramatically reduces the screw-like pumping effect.

The objective is not to achieve a smoother surface. The objective is to eliminate the helical lead responsible for lubricant migration. In other words, the grinding method directly influences the functional behaviour of the sealing surface.

How is the requirement verified?

Several inspection methods are available to verify the absence of excessive helical lead.

The thread test

Historically, one of the best-known methods is the thread test. Its principle is remarkably simple. A thin thread is placed on the shaft surface and kept under light tension by means of a small weight while the shaft rotates slowly. If the shaft has been properly plunge ground, the thread remains essentially in the same position. If microscopic helical grooves are present, the thread gradually moves along the shaft axis, following exactly the same path that lubricant would follow. The thread therefore acts as a simple indicator of the screw effect generated by the surface.

Although practical and easy to perform, this is a qualitative inspection method. It can reveal the presence of helical lead but cannot accurately quantify its magnitude.

Three-dimensional surface measurement

Today, inspection is generally performed using optical or contact profilometers capable of reconstructing the three-dimensional topography of the surface.

The software analyses the complete texture and determines:

groove direction;
helix angle;
helix pitch;
depth of the helical structure.

From these measurements, two key parameters are obtained. D, which defines the orientation of the microscopic grooves. Dt, which quantifies the depth of the helical component. This allows engineers to evaluate not only the direction of the machining marks but also the intensity of the screw effect.

Process validation rather than inspecting every part
In high-volume production, every shaft is rarely subjected to a complete helical lead measurement. Instead, manufacturers qualify the grinding process itself. They verify that the grinding machine, wheel, tooling and process parameters consistently produce compliant surfaces. Once the process has been validated, subsequent inspections become part of routine process monitoring. This approach ensures both manufacturing efficiency and consistent product quality.

Understanding D and Dt

Technical drawings for sealing surfaces often specify both D and Dt. Although they are closely related, they describe different characteristics.

D defines the orientation of the microscopic grooves. A value close to 0° indicates an almost perfectly circumferential surface texture, which is the preferred condition for preventing lubricant pumping.

Dt, on the other hand, represents the depth of the helical component present on the surface.

Together, these two parameters allow designers to control both the direction and the magnitude of the helical lead. Ultimately, they reduce the risk of lubricant leakage through the radial shaft seal. When a drawing specifies Plunge Grinding together with limits for D and Dt, the designer is not simply requesting a better surface finish.

For more information about mechanical transmission and gear solutions, contact GSI Ingranaggi.

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