The 58 mm housing diameter is the de-facto industry standard for industrial rotary encoder bodies, while the 38 mm class is a compact sub-segment used where axial space and shaft size are constrained, per manufacturer catalog data [S1][S3].
A typical 58 mm encoder ships with a case depth around 54 mm, hollow bore options of 14 or 15 mm, solid shaft options of 6 or 10 mm, and a 100 N maximum shaft loading rating (axial and radial) [S7]. A typical 38 mm class unit, exemplified by the Hengstler ACURO AD37, measures 38.3 mm in housing diameter, 29 mm in length, and supports 12,000 rpm continuous operation over -40 to +115 °C [S1].
Where Each Housing Size Fits in the Mechanical Stack
Industrial encoder housings are not interchangeable. The 58 mm form factor is the global baseline: Baumer lists 23 distinct 58 mm incremental encoder families in its 2020 selection guide, covering solid, blind hollow, and through-hollow shafts, with TTL/RS422, HTL/push-pull, and programmable outputs [S3]. The 38 mm class is narrower: a 38.1 mm housing (Hengstler AC36) is used when the application needs an absolute optical encoder with up to 22-bit singleturn resolution, BiSS or SSI output, and 10,000 rpm continuous operation over -40 to +100 °C [S1].
Smaller encoders also exist. The Hengstler RMB14 magnetic encoder module is built on a 14 mm PCB for miniature designs and tolerates 30,000 rpm, useful where the housing is integrated into a larger assembly [S1]. US Digital frames this as a general rule: bore size options range from 1.5 mm to 1 in. depending on the encoder family, so the housing diameter must be matched to the shaft it has to ride on [S4]. For background on how encoders read motion, see the linear encoder reference.
Comparison: 58 mm vs 38 mm on Decision Criteria
The two form factors trade off against each other on five criteria. Shaft and bore options: 58 mm units accept hollow shafts to 14-15 mm and solid shafts to 10 mm, with 100 N shaft loading [S7]; 38 mm units are built around smaller bores and lighter shaft loads. Speed: 38 mm optical absolute units are typically rated to 10,000-12,000 rpm [S1], while 58 mm incremental designs routinely handle higher mechanical speeds. Resolution: both classes can hit 22-bit singleturn optical resolution in absolute variants, so resolution alone does not pick a winner [S1]. Environmental: 58 mm Pepperl+Fuchs ENI58IL operates from -40 to +85 °C and supports high IP ratings for outdoor mounting [S2]; the 38 mm Hengstler AD37 extends further thermally to +115 °C [S1]. Mechanical envelope: 58 mm units run about 54 mm deep [S7], while 38 mm units such as the AD37 are 29 mm long, roughly half the axial footprint [S1].
For comparison, a 36 mm absolute encoder in the same Baumer catalog (EAM360 series) supports SSI, CANopen, analog, and Profinet outputs, with optional CX (C5-M) corrosion protection for offshore or washdown use [S3]. The 58 mm absolute class (EAM580, BMMV 58 hermetic) layers on the full bus-cover protocol set: SSI, CANopen redundant, Profinet, EtherCAT/EtherNet/IP, and Profibus [S3]. If the application is a hydraulic power unit where motor and encoder share a frame, the related AC induction vs DC motor on a hydraulic power unit article walks through the matching drive-side spec trade-offs.
Resolution, Output, and Protocol Coverage
Housing size does not determine resolution. Both 38 mm absolute optical units (Hengstler AC36, 22-bit singleturn via BiSS/SSI) and 58 mm absolute units (Hengstler AX73, 22-bit singleturn + 12-bit multiturn, optical geared) reach the same resolution ceiling; the AX73 extends the bus protocol set further with Profinet and EtherCAT variants [S1]. Output protocols for 58 mm Baumer families span TTL/RS422, HTL/push-pull, SSI, CANopen (including redundant and Lift variants), Profinet, EtherCAT, EtherNet/IP, and Profibus, with the integrated interface option and flexible bus cover as differentiators [S3].
Shaft and bore geometry is where size punishes you. A 6 mm shaft is the practical floor for many industrial 58 mm units; coupler internal diameter should be specified at 5-6.35 mm for a tight slip-fit and lower backlash [S6]. For 38 mm units, bore and shaft dimensions are scaled down accordingly, and the application will usually dictate a hollow-shaft or blind-hollow-shaft mount rather than a solid-shaft stub.
Use Case Mapping and Selection Rules
Pick 58 mm when the drive shaft is in the 6-15 mm range, the encoder must survive IP65-IP67 washdown or outdoor exposure, and the protocol set must include Profinet, EtherCAT, or SSI [S2][S3]. Pepperl+Fuchs positions the ENI58IL specifically for "all drive shafts up to 15 mm" with solid, hollow, and recessed hollow variants, an extended -40 to +85 °C range, and 6,000 rpm capability, making it the workhorse for small drives, spindles, machine tools, and milling machines [S2].
Pick 38 mm when the priority is a short axial envelope, the operating temperature can spike to +115 °C, and the application is a compact absolute position read rather than a high-speed incremental count [S1]. Hengstler AC36 and AD37 are the canonical 38 mm choices for that profile. If a 36 mm absolute fits the bore and the application needs redundant CANopen, the Baumer EAM360-R covers it with optional CX (C5-M) corrosion protection [S3]. For stepper-driven axes, the wiring-side trade-offs in 4-lead vs 6-lead vs 8-lead stepper motor wiring matter when the encoder feedback is on the same harness.
Limitations and Failure Modes by Size
58 mm housings carry real mechanical penalties: deeper case (about 54 mm) means a longer cantilever off the motor face, which interacts badly with the 100 N shaft loading limit if radial force is applied too far from the bearing [S7]. 38 mm units trade the depth penalty for thermal headroom but cap continuous speed at 10,000-12,000 rpm and limit bore diameter, so forcing a 15 mm shaft into a 38 mm body is not an option [S1].
Encoder size names do not always mean what they seem. Hobby-class 11 mm and 12 mm encoders differ only in a 0.7 mm body width (11.7 mm vs 12.4 mm) on datasheets, and both share a 6 mm shaft diameter; the number is closer to a model code than a real mechanical constraint [S5]. US Digital also notes that bolt-circle options (e.g. 0.586 in. vs 0.750 in. on the M3K miniature) are a separate variable from housing diameter and must be selected alongside bore size [S4]. Skipping that step is the most common reason an encoder "fits the shaft but not the mount."
Verifiable Signals to Track
Two signals are worth watching. First, Baumer's EAM360 (36 mm) and EAM580 (58 mm) absolute families remain in the 2020 selection guide with parallel bus-protocol coverage (SSI, CANopen, Profinet, EtherCAT, EtherNet/IP, Profibus), so a future catalog revision that drops the 36 mm class would compress the 38 mm sub-segment into a 58 mm-or-smaller split [S3]. Second, the Pepperl+Fuchs BlueBeam ENI58IL product line continues to be marketed against the 15 mm shaft ceiling, with the ENI90 (90 mm) family covering the next tier up to 38 mm hollow shafts [S2], making 58 mm the default size for any drive above miniature but below heavy-duty.
Detailed specification references: rotary hammer.