Strength check

Generated by python -m strido_verify.strength from hardware/strength.yaml, the printed STLs, the leg geometry and the fit report. 19 checks: 19 ok, 0 warn, 0 fail; lowest safety factor 2.22 (tibia halves bending (knee torque, shared by the two halves)). Safety factor S = allowable / actual; fail below 1.5, warn below 2.0. Formulas and background: strength, torque and failure.

Loads

Load	Value	How
Foot force, hard landing	45.2 N	F = 3.0 · m · g / 2, m = 3.07 kg
Thigh and hip joint torque	3.43 N·m	servo stall torque (the largest torque the servo applies)
Knee joint torque	3.07 N·m	knee servo at stall through the linkage, largest over the joint range
Knee torque a landing would need	5.60 N·m	F_foot · L_tibia (124 mm); beyond the servo's torque the knee is back-driven
Short link force	211 N	T_stall / (r_arm · sin α), largest over the joint range (r_arm 24.0 mm)
Long link force	192 N	T_crank / (r_crank_b · sin β), largest over the joint range
Knee joint radial force	237 N	F_foot + F_long (upper bound)

Material: PETG (Prusament PETG technical data sheet v1.1, printed specimens, 0.2 mm layers): 47 MPa along the layers, 18 MPa across them. Sections count only the 1.2 mm walls (infill ignored). Screws: DIN 912 A2-70 (ISO 3506, Bossard tables).

Checks

Check	Parts	Formula	Actual	Allowable	S	Status
femur bending at its weakest section (thigh servo stall torque)	femur	$\sigma =T_{\mathrm{thigh}}/W$	14.54 MPa	47 MPa	3.23	ok
tibia halves bending (knee torque, shared by the two halves)	inner_tibia, outer_tibia	$\sigma ={\displaystyle \frac{T_{\mathrm{knee}}}{W_{\mathrm{inner}}+W_{\mathrm{outer}}}}$	21.16 MPa	47 MPa	2.22	ok
short link tension/compression (pin-line offset e = 0.0 mm)	short_link	$\sigma ={\displaystyle \frac{F}{A}}+{\displaystyle \frac{Fe}{W}}\sec \left({\displaystyle \frac{L}{2}}\sqrt{{\displaystyle \frac{F}{EI}}}\right)$	8.65 MPa	47 MPa	5.44	ok
short link buckling (pinned in the leg plane, K = 0.7 across it)	short_link	$F_{\mathrm{cr}}={\displaystyle \frac{{\pi }^{2}EI}{(KL{)}^2}}$	211.49 N	2002.8 N	9.47	ok
short link pin hole bearing pressure	short_link	$p={\displaystyle \frac{F}{dt}}$	4.28 MPa	47 MPa	10.97	ok
long link tension/compression (pin-line offset e = 2.0 mm)	stiff_long_link	$\sigma ={\displaystyle \frac{F}{A}}+{\displaystyle \frac{Fe}{W}}\sec \left({\displaystyle \frac{L}{2}}\sqrt{{\displaystyle \frac{F}{EI}}}\right)$	10.82 MPa	47 MPa	4.34	ok
long link buckling (pinned in the leg plane, K = 0.7 across it)	stiff_long_link	$F_{\mathrm{cr}}={\displaystyle \frac{{\pi }^{2}EI}{(KL{)}^2}}$	192.1 N	486.6 N	2.53	ok
long link pin hole bearing pressure	stiff_long_link	$p={\displaystyle \frac{F}{dt}}$	6.61 MPa	47 MPa	7.11	ok
4 × M3 shear, femur on the thigh horn (r = 7.0 mm)	femur, femur_servo_connector	$\tau ={\displaystyle \frac{T}{nr{A}_s}}$	24.35 MPa	259.6 MPa	10.66	ok
printed hole bearing pressure, femur on the thigh horn	femur, femur_servo_connector	$p={\displaystyle \frac{T}{nrdt}}$	8.48 MPa	47 MPa	5.55	ok
2 × M3 shear, hip horn on the servo mount (r = 7.3 mm)	left_servo_mount, left_servo_mount_fl, right_servo_mount, right_servo_mount_fr	$\tau ={\displaystyle \frac{T}{nr{A}_s}}$	46.51 MPa	259.6 MPa	5.58	ok
printed hole bearing pressure, hip horn on the servo mount	left_servo_mount, left_servo_mount_fl, right_servo_mount, right_servo_mount_fr	$p={\displaystyle \frac{T}{nrdt}}$	17.93 MPa	47 MPa	2.62	ok
M3 shear, arm pin (short link)	short_link	$\tau =F/A_s$	42.04 MPa	259.6 MPa	6.18	ok
M3 shear, tibia link pin (long link)	inner_tibia, outer_tibia, stiff_long_link	$\tau ={\displaystyle \frac{F}{2A_s}}$	19.1 MPa	259.6 MPa	13.6	ok
M3 shear, knee axle screw (one per side)	femur, inner_tibia, outer_tibia	$\tau =F/A_s$	23.59 MPa	259.6 MPa	11.01	ok
knee axle bearing pressure in the femur pilot	femur	$p={\displaystyle \frac{F}{d{L}_e}}$	10.59 MPa	47 MPa	4.44	ok
633ZZ static radial load (knee, two per leg)	bearing_633zz	$S_0=C_0/F_r$	118.64 N	490 N	4.13	ok *
6805-2RS static radial load (shoulder crank)	bearing_6805	$S_0=C_0/F_r$	403.58 N	2600 N	6.44	ok *
625ZZ static radial load (hip pivot)	bearing_625zz	$S_0=C_0/F_r$	45.18 N	380 N	8.41	ok *

* 633ZZ static radial load (knee, two per leg): Koyo 633 ZZ

* 6805-2RS static radial load (shoulder crank): SKF 61805-2RS1

* 625ZZ static radial load (hip pivot): SKF 625-2Z

Tightening torque into printed pilots

Pull-out of a thread-forming screw shears the plastic on a cylinder of the thread diameter: $F=\tau \pi d{L}_e$ with $\tau =0.577\times$ layer adhesion (the weakest direction). The torque that produces it is $T=kFd$ ($k=0.2$); tighten to at most half of that.

Joint	Part	Thread	Engaged	Pull-out force	Stripping torque	Tighten to max.
bracket -> front shell boss (self-tapping)	front_shell_display	M3	6.5 mm	636 N	0.38 N·m	0.19 N·m
bracket_screws	left_servo_mount	M3	12.0 mm	1175 N	0.7 N·m	0.35 N·m
bracket_screws	left_servo_mount_fl	M3	12.0 mm	1175 N	0.7 N·m	0.35 N·m
bracket_screws	right_servo_mount	M3	12.0 mm	1175 N	0.7 N·m	0.35 N·m
bracket_screws	right_servo_mount_fr	M3	12.0 mm	1175 N	0.7 N·m	0.35 N·m
bracket_screws	servo_mount_top_bracket	M3	7.0 mm	685 N	0.41 N·m	0.21 N·m
bus holder -> lower shoulder frame (self-tapping)	lower_shoulder_frame_front	M3	5.0 mm	489 N	0.29 N·m	0.15 N·m
bus holder -> lower shoulder frame (self-tapping)	lower_shoulder_frame_rear	M3	5.0 mm	489 N	0.29 N·m	0.15 N·m
clamp ring -> bezel through the D20 hole	arming_port_bezel	M2.5	2.5 mm	204 N	0.1 N·m	0.05 N·m
crank_pins	left_shoulder	M3	11.5 mm	1126 N	0.68 N·m	0.34 N·m
crank_pins	right_shoulder	M3	11.5 mm	1126 N	0.68 N·m	0.34 N·m
hip_horn	left_servo_mount	M3	4.5 mm	440 N	0.26 N·m	0.13 N·m
hip_horn	left_servo_mount_fl	M3	4.5 mm	440 N	0.26 N·m	0.13 N·m
hip_horn	right_servo_mount	M3	4.5 mm	440 N	0.26 N·m	0.13 N·m
hip_horn	right_servo_mount_fr	M3	4.5 mm	440 N	0.26 N·m	0.13 N·m
hip_pivot	left_servo_mount	M3	4.0 mm	392 N	0.23 N·m	0.12 N·m
hip_pivot	left_servo_mount_fl	M3	4.0 mm	392 N	0.23 N·m	0.12 N·m
hip_pivot	right_servo_mount	M3	4.0 mm	392 N	0.23 N·m	0.12 N·m
hip_pivot	right_servo_mount_fr	M3	4.0 mm	392 N	0.23 N·m	0.12 N·m
knee_axle	femur	M3	4.0 mm	392 N	0.23 N·m	0.12 N·m
leg_servo_ears_and_bracket	left_servo_mount	M3	9.5 mm	930 N	0.56 N·m	0.28 N·m
leg_servo_ears_and_bracket	left_servo_mount_fl	M3	9.5 mm	930 N	0.56 N·m	0.28 N·m
leg_servo_ears_and_bracket	right_servo_mount	M3	9.5 mm	930 N	0.56 N·m	0.28 N·m
leg_servo_ears_and_bracket	right_servo_mount_fr	M3	9.5 mm	930 N	0.56 N·m	0.28 N·m
pedestal -> top shell (from below, self-tapping)	lidar_pedestal	M3	9.5 mm	930 N	0.56 N·m	0.28 N·m

Limits

• Static checks with peak loads; fatigue of PETG under millions of steps, creep under a standing load and notch effects at sharp corners are not modelled (hence S ≥ 2).
• Joint torques are the servos' rated stall torque. Back-driving a powered spur-gear servo from outside takes more (about 1 / (η_forward · η_reverse), roughly 1.5–3 × for a multi-stage gearbox [estimate]): a hard impact can exceed these loads.
• The impact factor is an assumption, not a measurement; the servo stall torque is the datasheet value at 6.0 V.
• Bending is taken in the leg plane at the weakest of 41 sections; buckling covers both directions with the stiffness of the sections in series. Bending of the tibias across the leg plane (side loads) is not checked: the two halves are joined, and their combined section is not modelled.
• Hole bearing pressure is compared with the strength along the layers.
• The aluminium horns are not checked (their alloy is not stated by the sellers); they are far stronger than the printed parts they hold.